In a microwavable bowl, mix glutinous rice flour, sugar, milk, and food coloring (if using) until the batter is smooth with no lumps.

Cover the bowl tightly with plastic wrap or a microwave-safe plate, leaving a small gap for steam to escape. Microwave on high for 1 minute.

Carefully remove the bowl (it will be hot) and stir the mixture with a wet spatula or spoon. It should be semi-translucent and thickening. If there is a grainy mouthfeel, continue cooking.

Cover again and microwave for another 1-2 minutes, checking and stirring every 30 seconds, until the mochi is glossy, sticky, and no longer liquidy. It should look like a soft, pliable dough.

Generously dust a clean work surface with cornstarch. Carefully transfer the hot mochi dough onto the dusted surface. Dust the top of the mochi and your hands with more cornstarch.

Using a rolling pin or your hands, gently flatten the mochi dough to about 1/4 inch thickness. Be careful, as it will still be warm and sticky.

Using a knife or cookie cutters, cut the mochi into desired shapes or squares. Alternatively, you can pinch off small portions and shape them into balls, filling them if desired.

Serve immediately while still warm and chewy. Store any leftovers in an airtight container at room temperature for up to one day.