Practice makes perfect! Don’t be put off if your mochi are not perfect the first time around, these Japanese treats take time to master. Stick with it and before you know it you will be folding up your mochi like a pro. Plus, they will be delicious no matter what they look like.
Line a small baking sheet or an egg carton with plastic wrap. Scoop the ice cream into 8 tight round balls using a 1-ounce cookie scoop (about 2 packed tablespoons each). Freeze on the prepared pan or in the egg carton cups until solid, at least 4 hours or overnight.
Place a large piece of parchment paper on a work surface. Using a fine-mesh sieve, dust the parchment generously with cornstarch. (Use more cornstarch than you think you need — the dough will be very sticky.) In a large bowl, whisk the rice flour, sugar, water and food coloring until smooth. Pour the mixture into a small saucepan and cook over medium heat, stirring and folding with a rubber spatula, until a stiff tacky dough forms, 5 to 7 minutes. (It’s OK if it looks lumpy at first; it will smooth out.) The dough should be very thick and feel slightly tough and rubbery.
Transfer the dough to the parchment and dust the top generously with cornstarch. Place another piece of parchment over the dough and carefully press into a thin disk. Let the dough cool to room temperature, about 1 hour.
Roll out the dough into a large round or oval, about 1/4 inch thick. Using a 4- to 4 1/2-inch fluted or plain round cookie cutter, cut out as many rounds as possible. Gather the scraps to cut out more rounds, dusting with more cornstarch as needed (you should have at least 8 rounds). Dust a plate with cornstarch; make sure the dough rounds are dusted with cornstarch, then stack them on the plate and cover with plastic wrap. Refrigerate at least 30 minutes.
Assemble the mochi: Use plastic gloves if you have them. Line a small baking sheet or a 9-by-13-inch baking dish with parchment paper, dust generously with cornstarch and place in the freezer. Fill a small dish with cool water. Working with one dough round at a time, gently brush off any excess cornstarch and place the round flat in your hand. Working quickly, place a scoop of ice cream into the center of the dough. Gently pull up the edges of the dough to enclose the ice cream, using a small dab of water if the seams aren’t sticking. Gently stretch, twist and pinch the dough together at the seams to secure. Immediately place the mochi seam-side down in the cold pan in the freezer. Repeat with the remaining dough rounds and ice cream balls. Cover the mochi with plastic wrap and freeze until firm, 1 to 2 hours. Let soften for a few minutes before eating.