In a food processor or blender, finely grind the sesame seeds. Add the caster sugar and use a spatula to mix together in the food processor a bit. Then add the butter and pulse for 10 seconds at a time until the mixture is well combined. Scoop everything out into a bowl and put in the refrigerator for about 30 minutes. The purpose of this cooling process is to harden the mixture slightly for easy handling later.
While the filling is cooling, make the dough. Put the sweet rice flour into a mixing bowl. Slowly add in the warm water, while stirring to form a dough. The dough can’t be too wet, or it will not hold its shape. Cover with a damp towel.
Now boil some water in a small pot, take out a small piece of the dough, about 1” in diameter. Add it to the boiling water and cook the piece of dough until it floats. Scoop it out and add it to the rest of the dough. Mix the two parts together until the dough is soft and smooth. Divide the dough into 12 equal pieces (about 18 grams each, if you want to be really exacting), and roll them into balls. Keep the dough and dough balls covered with a damp paper towel while assembling.
Take out the filling after it’s cooled for 30 minutes. It should be slightly hard, but still workable. Roll the filling into small marble-sized balls, about 4 to 5 grams each. Then take each dough ball and flatten into a circle about 2 inches in diameter. Add the filling to the center, and close the dough over the filling. Roll the ball around in your hands until it's smooth and round. Repeat until all the sesame balls are made. Cover the remaining filling and return to the refrigerator for later!
While you are assembling, bring a medium pot of water to a boil. Add the sesame balls and stir immediately to prevent them from sticking. Adding the sesame balls will cool down the water temperature significantly, so once the water comes back up to a boil, add a 1/2 cup of water to the pot. Repeat this step two more times. This cooking process will allow enough time for the sugar and butter to melt nicely into a smooth filling. Serve the sesame balls in a bit of the hot cooking water!