1. 1. In a large bowl, combine the glutinous rice flour and water. Knead the mixture until it forms a smooth, pliable dough.

2. 2. Divide the dough into small, equal-sized balls, about 1-inch in diameter.

3. 3. For the filling, mix the sugar and black sesame seeds (if using) in a small bowl.

4. 4. Gently flatten each dough ball and place a small amount of the filling in the center. Carefully wrap the dough around the filling to enclose it completely.

5. 5. Bring a large pot of water to a boil. Carefully drop the filled dough balls into the boiling water and cook for 5-7 minutes, or until they float to the surface.

6. 6. Using a slotted spoon, transfer the cooked Tang Yuan to a serving bowl. Serve warm, or chill in the refrigerator until ready to enjoy.