Toast ⅔ cup of black sesame seeds in a pan until they plump. Grind with 2½ tablespoons of sugar into a paste, then mix in 2½ tablespoons of softened butter.
Chill the filling until firm, then roll it into 20 small balls.
Stir 3 tablespoons of boiling water into 1 cup of glutinous rice flour. Gradually knead in 4 tablespoons of room-temperature water to make a soft dough that doesn't stick.
Divide the dough into 20 pieces and roll them into balls. Flatten each one, wrap it around a ball of filling, and roll smooth.
Bring a large pot of water to a full boil. Add the balls and cook over medium-high heat; once they float, boil for 1 minute more.
