Combine the black sesame seeds and sugar in a food processor. Blend until the sesame seeds are finely ground.

Add the softened butter to the food processor. Pulse until the butter is evenly blended. Transfer the mixture onto a plate. Use a spatula to press the filling into a thin layer so it hardens faster. Cover the plate and refrigerate or freeze until the butter is solidified enough to handle, a minimum of 30 minutes in the fridge or 15 minutes in the freezer.

To make the sweet rice ball filling, scoop out about 1/2 tablespoon of the filling (7 g). Use your palms to roll the filling until it forms a round ball. Keep working to make the filling balls. If the butter becomes too soft, transfer it back to the fridge to solidify it. Once done, place the plate of black sesame balls back in the fridge or freezer (*Footnote 1).

Add the glutinous rice flour to a medium-sized bowl and pour in the water. Mix until a dough forms. Sprinkle some flour onto a working surface and dust your hands with it. Transfer the dough onto the working surface. Use your hands to knead the dough a few times until it’s smooth and soft.(*Footnote 2) Return the dough to the bowl and cover with plastic wrap when not using.

Take about 1 scant tablespoon (16 g) of the dough. Use your palms to shape the dough into a round ball. Then use your thumb to press the center of the dough so it forms a small bowl shape. Place the black sesame ball in the center of the dough. Use your thumb to press the filling, while using your other hand to slowly smooth and stretch the dough over the filling. When the dough fully covers the filling, pinch the dough together to seal it. Then roll the rice ball between your hands so it’s perfectly round. Then place it onto a tray and work on the rest of the rice balls.

Once you’ve shaped the rice balls, you can freeze them and cook them later. Or you can cook them immediately.

Bring a medium-sized pot of water to a boil over medium-high heat. Add the rice balls according to the number of servings you want to make (*Footnote 3). Stir gently to prevent them from sticking to the pot. Cook until the water comes to a boil again, then turn to medium heat. Once the rice balls have risen to the top, keep boiling for another 2 minutes. Then transfer the rice balls along with some boiling water to the serving bowls. The boiling water should just cover the rice balls. Serve hot and enjoy!