Roll the red bean paste into 8 equal balls (about 1 tablespoon each) and set aside on a plate. Generously dust a large work surface and a separate plate with potato starch.
In a microwave-safe bowl, whisk together the sweet rice flour, sugar, and water until completely smooth with no lumps remaining.
Cover the bowl loosely with plastic wrap and microwave on high for 2 minutes. Remove and stir vigorously with a wet spatula, then microwave for another 1 minute until the dough is translucent and glossy.
Immediately turn the hot mochi dough onto the prepared starchy surface. Dust the top heavily with more potato starch and let cool for 2-3 minutes until safe to handle.
Lightly oil your hands, then divide the mochi into 8 equal pieces using a bench scraper or knife. Work with one piece at a time, keeping others covered.
Flatten one mochi piece into a 3-4 inch circle in your palm. Place a red bean ball in the center and gather the edges upward, pinching and stretching the dough to seal completely at the top.
Twist off any excess dough at the seam, then flip the mochi smooth-side up and gently roll between your palms to create a round shape. Place seam-side down on the starchy plate.
Repeat with remaining pieces, dusting your hands with potato starch as needed. Brush off excess starch with a soft pastry brush before serving.
