In a microwave-safe bowl, combine the glutinous rice flour and granulated sugar. Whisk together until evenly distributed. If adding matcha or cocoa powder, whisk it in now.

Gradually pour in the milk (or water) while stirring continuously. Add the white vinegar if using. Stir until the mixture is completely smooth and free of lumps.

Cover the bowl loosely with plastic wrap or a microwave-safe plate. Microwave on high for 1 minute.

Remove the bowl carefully and fold the mixture with a wet spatula. It will look partially cooked and lumpy; that's normal. Cover again and microwave for another 1 minute.

Fold again. The dough should now be thick, sticky, and slightly translucent. If it still looks raw in spots, microwave in 15-second increments, folding between each, until the dough is glossy, elastic, and pulls away from the sides of the bowl.

Generously dust a clean work surface and your hands with cornstarch.

Scoop the warm dough onto the dusted surface. Divide into 10-12 equal pieces using a spatula or your hands.

For plain mochi: Roll each piece into a ball, dust lightly with cornstarch, and place on a plate.

For filled mochi: Flatten each piece into a 3-inch circle. Place about 1 teaspoon of filling in the center. Gather the edges up and pinch them together to seal completely. Roll gently between your palms to smooth, then place seam side down on a cornstarch-dusted plate.