Heat the sugar and water in a saucepan over medium heat. Stir to dissolve the sugar. Once boiled, stir in the lemon juice, and immediately reduce the heat to low. Let the syrup simmer uncovered for 60-70 minutes (do not stir!), until the sugary liquid turns a light amber color.
After 60-70 minutes, the liquid might look a bit watery, but it will thicken once cooled. Try not to overcook it, because it will become thick like caramel (it should be the consistency of honey). If you do overcook the syrup and it’s too thick, add a few drops of hot water to thin it out. Keep adding hot water until you get a honey-like consistency.
Pour the sugary liquid into a heatproof container, and let it cool completely before storing it in the refrigerator.
Making the Lotus Paste:
Using a butter knife, gently split the lotus seeds in half through the top opening. Remove and discard the bitter green center. Rinse and soak them overnight in a large bowl, covered with at least 2 inches/5 cm of water.
The next day, wash and drain the lotus seeds. In a medium pot, add the lotus seeds and 4 to 4 1/2 cups of water (the water level should be above the lotus seeds). Bring the mixture to a boil over high heat. Once the water reaches a boil, reduce the heat to low, and simmer for 30 minutes. It’s done once the lotus seeds are softened but have maintained their shape.
Turn off the heat, let cool, and then drain the liquid in to a separate bowl. (Set this liquid aside.) In a food processor, puree the cooked lotus seeds (it’s best to do this in two batches). Slowly add some of the reserved cooking liquid until the puree is very smooth but not too runny (a hummus-like consistency).
Transfer the lotus puree to a non-stick or well-seasoned cast-iron pan). Turn the heat to medium/medium-low (it shouldn’t be so high that it generates steam or smoke at any point). Stir/flip/fold the puree constantly using a silicone spatula for about 30 minutes to avoid crusting. During this process, add the oil in 3-4 batches. Then stir in the sugar in 3-4 batches. Lastly, add the maltose in 2 batches. When adding each batch of material, make sure the last batch was well incorporated (2-3 minutes) before adding the next batch.
The lotus filling is done once the paste can holds its shape while also being pliable. Remove from the heat and cool completely before storing in an airtight container in the refrigerator. You can make this 1 day in advance, as it’s easier to work with when the filling has been completely chilled.
Making the salted duck egg yolks:
There are a few ways to prepare the salted duck egg yolks: 1. You can buy raw salted duck eggs, crack the eggs open, and take out the yolks. Gently rinse the yolks under a small stream of running water to wash off any residual egg white. Preheat the oven to 350°F/175°C. Dip each yolk briefly in baijiu (白酒, a clear Chinese liquor) just to coat it, then put the egg yolks on a parchment-lined baking sheet. Bake for 8-10 minutes, let them cool completely, and they are ready to use. You can also prepare this ahead of time. Just make sure to store them in an airtight container and in the fridge.2. You can buy pre-extracted, cooked salted duck egg yolks, like the ones I use in this recipe. I have found these to be slightly drier than I would normally like, but they are convenient. Bake on a parchment-lined baking sheet at 350°F/175°C for 8 minutes. When they're cooled completely, they're ready to use. You can also buy whole cooked salted duck eggs, and extract the whole yolk. These are ready to use. No baking needed. You can also prepare this ahead of time...just make sure to store them in an airtight container in the fridge.
Making the dough:
First mix the sugar syrup, oil, and lye water together. Then add the mixture to the flour, and use a rubber spatula to fold and combine everything into a soft dough. Do not overwork it. This step should only take about a minute or two.
Cover the dough in an airtight container or with an overturned plate. Let it rest in the refrigerator for 1 hour. Chilled dough is much easier to work with. (Side note: you may also leave the dough in the refrigerator overnight if you'd like to make it ahead of time.)
Assembling the mooncakes:
While the dough is resting, start assembling the filling. My mooncake mold was marked 100 grams, which means each mooncake should weigh about 100 grams total after assembly. This recipe makes 18 mooncakes total: 12 mooncakes with salted duck egg yolks and 6 mooncakes without.
Separately weigh out twelve 58-gram scoops of lotus filling and six 70-gram scoops of lotus filling. Shape each portion of filling into balls. Next, take one of the 58 gram lotus balls, make a deep well in the middle, and insert one salted duck egg yolk into the middle. Then close the top and re-shape it into a ball. Repeat for the remaining 11 pieces. It’s good to separate the “with yolks” and the “without.” Cover and refrigerate until you are ready to assemble the rest of the mooncakes.
Once the dough has finished resting, it’s time to prepare the mooncake mold. My mold came with four pattern plates––one plate should be attached to the mold itself and locked in place. Wrap another one with clear plastic, because we will use it to press the mooncake from the bottom. Dust the inside of the mold with plenty of flour and shake off the excess. You’ll need to repeat this process before pressing every mooncake to avoid sticking. Divide and weigh the dough into eighteen 25-gram pieces, plus 1 smaller dough ball for scraps/mending. Shape each piece into a ball, and lightly dust the dough balls with flour. (It's best to keep most of the dough balls chilled in the refrigerator while assembling, and only work with a couple at a time.)
Take one dough ball, lightly dust it with flour, and roll it out on a lightly floured surface into a 4 inch/10 cm round. Put one piece of filling in the center. Carefully lift it up and turn it upside down, and gently press out the air bubbles around the lotus ball, without breaking the dough. Now turn the opening facing up, and slowly press the dough together to close the opening, keeping the distribution of the dough as even as possible. The dough feels like Play-doh, so it’s easy to break, but it’s also easy to squish back together. Keep in mind that the chief goal here is to close the dough around the filling without capturing air inside.
Now lightly dust the assembled ball with additional flour. Place it inside the mooncake mold with the mold opening facing up. Now press in the bottom piece (with the clear plastic-wrapped plate that you prepared). With some pressure, gently press it inward until it stops giving. Quickly sit the mold down, and press the mooncake out with the top pressure bar. The bottom piece will come out no problem, but use your other hand to VERY GENTLY guide the mooncake out of the mold.
If/When a small piece of dough gets stuck and does not come out, get a small piece of the “scrap”/“mending” dough to patch up the area. Then clean the mold (a toothpick helps get the dough out of any nooks and crannies in the mold) and dust it well with flour again. Then gently flatten all sides of the mooncake to make it smaller, so it can go back in the mooncake mold for re-shaping. Use slightly more flour for dusting if this becomes a persistent issue. Place the mooncakes on a baking sheet as you make them. Repeat until you've assembled all 18 mooncakes. (Note that they will not change size during baking.)
Baking the mooncakes:
Preheat the oven to 325°F/160°C when you are almost finished assembling the mooncakes. Once the oven is preheated, gently mist the mooncakes with a food-grade spray bottle (if you have one) filled with water. You can also dip your fingers into a bowl of water, and fling the water on your fingers onto the mooncakes a couple of times. This prevents the dough from cracking.
Immediately put the mooncakes in the oven and bake for 5 minutes. (Remember to set the timer!) Now prepare the egg wash by whisking the egg yolk and water together. After the mooncakes have been baking for 5 minutes, take them out of the oven and immediately lower the oven temperature to 300°F/150°C. Lightly brush each mooncake with egg wash, and put them back in the oven. Bake for another 15 minutes (remember to set the timer!)
When the mooncakes first come out of the oven, the dough will look dry like bread, and not oily like what you are used to when opening packaged mooncakes. This is normal. Cool completely, and then store the cooled mooncakes in an airtight container for 1-2 days, and the outside will get that nice shiny sheen you recognize!
Makes 18 mooncakes (12 with egg yolks, 6 without). Store in the refrigerator in an airtight container for up to 10 days.