Start by making the filling. Melt the butter in a medium skillet over medium heat. Add 2 cups pumpkin puree, 1/3 cup granulated sugar, and 1/2 teaspoon cinnamon. Stir often and cook until the mixture darkens and comes together into a thick paste. This process will take about 30 minutes. Cool completely.
Then you can make the dough. While the filling cools, sift together the sweet rice flour, cornstarch, and caster sugar in a mixing bowl. Add the remaining 1 1/2 cups pumpkin puree and mix to form a smooth dough. The dough should be pliable. If it cracks when handling, it is too dry. Add a couple drops of water. If the dough is too wet, add more sweet rice flour.
Divide the dough into 15 equal pieces and roll them into balls. Cover with a damp paper towel. Once the filling is cooled, also divide it into 15 equal pieces and roll individually into balls.
To assemble, rub a couple drops of water into your palms to make the dough easier to handle. Take a dough ball, flatten it into a 3” circle, and add a ball of filling in the middle. Close the dough over the filling and roll into a smooth ball. Press the ball down lightly so it's shaped almost like a flat pumpkin. Then use a toothpick to press a pattern of outer lines into your pumpkin shape. Top with a clove as your pumpkin's "stem."
Prepare your steamer with cold water and line the rack with cheese cloth. Place the mini pumpkins on the steamer rack 2 inches apart. Start with cold water and steam for 10-12 minutes using high heat. The cooked pumpkins should look somewhat translucent.