Begin by preparing the peanut butter balls. Line a baking sheet with plastic wrap or parchment paper. Using a melon scoop, scoop out 12 balls of peanut butter. If they are not nicely shaped, roll them lightly between your hands. Freeze for at least 2 hours, or until very firm.
Place the rice flour, sugar, and water in a microwave safe bowl and mix until smooth. The batter will be quite loose. Incorporate the food coloring just before you cook the dough.
Place the bowl in the microwave and heat on high for 30 seconds at a time, stirring well after each time. Depending on your microwave, it should take between 3 to 4 minutes (but it's essential to work in 30-second bursts) for the batter to fill with air and swell and then deflate immediately. It will look a bit like chewing gum.
Generously dust a work surface or large plate with potato starch sifted through a small sieve. Transfer the dough to the surface and sprinkle it with potato starch. You're going to be handling the dough while it is still hot and the potato starch protects your hands.
Break off enough dough to make a ping-pong sized ball and use your other hand to insert the ball of frozen peanut butter in the center of the mochi dough through the tear at the point where you broke it off. Close up the mochi by joining the dough around the peanut butter, and ensure that it is sealed. Roll the ball between your palms, adding a little potato starch if necessary, to make a nice shape.
Dust a baking sheet with potato starch and place the mochi on it. If there is too much potato starch on them, brush it off gently with a soft pastry brush. Flatten the balls slightly between the palms of your hands. Repeat with the remaining dough.
Serve the same day, or store in an airtight container, tightly wrapped with plastic wrap, for an additional day.