Line a small baking sheet with parchment paper. Scoop 12 tablespoon-sized balls of peanut butter onto the sheet and freeze for 30 minutes until firm.
In a large microwave-safe bowl, whisk together the sweet rice flour and sugar until combined. Add the water and melted butter, stirring until completely smooth with no lumps.
Cover the bowl loosely with plastic wrap and microwave on high for 1 minute. Stir the mixture, then microwave for another 30 seconds. The dough should be translucent, stretchy, and pull away from the sides of the bowl.
Dust a clean work surface heavily with cornstarch. Turn the hot mochi dough out onto the surface and dust the top generously with more cornstarch.
Using a cornstarch-dusted knife or bench scraper, divide the dough into 12 equal pieces. Work quickly while the dough is still warm and pliable.
Flatten one piece of dough into a 3-inch circle in your palm. Place a frozen peanut butter ball in the center and gather the edges up and around the filling, pinching firmly to seal.
Roll the sealed mochi between your palms to smooth it into a ball, then roll in cornstarch to coat completely. Repeat with remaining pieces.
Let the finished mochi rest at room temperature for 15 minutes before serving, allowing the texture to soften to perfect chewiness.
