In a medium microwave-safe bowl, whisk together the mochiko and sugar until no lumps remain. Add the water and vanilla extract if using, stirring until you have a completely smooth, thin batter.
Cover the bowl loosely with plastic wrap, leaving one corner open to vent. Microwave on high for 1 minute, then remove and stir vigorously with a wet spatula to distribute heat evenly.
Microwave for another 1 minute, stir again, then microwave for a final 30 seconds. The mochi is done when it appears translucent, glossy, and slightly puffed with no white streaks remaining.
Generously dust a clean work surface or large plate with potato starch. Turn the hot mochi dough out onto the starch and dust the top surface heavily as well.
Once cool enough to handle (about 2-3 minutes), coat your hands with starch and divide the dough into 12-15 equal pieces using a bench scraper or knife dusted with starch.
If filling, flatten each piece into a 3-inch circle in your palm. Place 1 teaspoon of filling in the center, gather the edges upward, and pinch firmly to seal, rolling gently to smooth.
For unfilled mochi, simply roll each piece into a smooth ball. Dust off excess starch with a dry pastry brush and serve immediately or store in an airtight container.
