Make the filling: In a medium bowl, combine the ground black sesame seeds, powdered sugar and melted butter. Mix until well combined. Roll 1 tablespoon scoops of the mixture into balls. Set aside.
Make the dough: Add the rice flour to a medium bowl. Gradually add the water and mix with your hands until the dough comes together.
On a lightly floured surface, turn out the dough. Shape into a ball and cut in half. Roll the dough into 2 logs, then cut into about 16 2-3-ounce (55G -85G) portions.
Flatten a dough portion in your palm and add a sesame ball to the center. Encase the filling with the dough and roll into a ball. Repeat with the remaining ingredients.
Bring a large pot of water to boil. Add the sugar and osmanthus flower. Drop the dumplings into the boiling water. Gently stir so they don’t stick to the bottom of the pot. Cook until the dumplings float to the surface, then remove from the pot.
Serve the dumplings in a bowl with the cooking liquid.