Toast Black Sesame Seeds (1 cup) in a pan or oven (detailed guide can be find here) and then ground them into powders. Smaller particles can be accepted for the filling. Or you might find them directly in store.
In a bowl, mix black sesame powder with Salt (1/4 teaspoon) and Powdered Confectioners Sugar (1 cup).
Pour Unsalted Butter (150 milliliter) and mix well. Place in fridge for 1 hour until slightly hardened.
Place the Sweet Glutinous Rice Flour (2 cup) into a large bowl, add Water (1/4 cup) in the center and wait for 10 minutes. Then pour in the Water (3/4 cup).
Grasp to form a dough and keep kneading for couple of minutes until the smooth; shape the dough into a long log so that you can make similar pieces in the following step.
Take one piece and shape it to a bowl, Then scoop around 1/2 tablespoon - 1 tablespoon toasted black sesame filling into the center. Seal the rice dumpling completely and as round as possible.
Put them in a large pot with water to cover them. Bring to a boil, and continue cooking for another 1-2 minutes after the tang yuans have floated to the surface.
Scoop out with the soup and serve with sugar, sweet osmanthus or syrup.