Preheat the oven to 450 degrees Fahrenheit.
Cut the mochi into 2-inch squares. (Smaller squares are difficult to stuff, but they're fine if you're eating the mochi plain or in a soup.)
Place the mochi on a baking sheet and bake for 8 to 10 minutes, until mochi bubble up significantly. Remove from heat and let cool.
Use a small spoon to poke a hole at the side of the mochi where the bubbled part meets the flat part. Use a spoon or small funnel to pour the sauce, pudding, spread, or ice cream into the mochi.
Serve at room temperature or frozen, depending on stuffing type.