Slowly whisk in water until smooth. Cover the bowl with plastic wrap or a microwave-safe lid. Microwave on high for 1 minute, stir, then microwave in 30-second intervals (usually 2–3 more times), stirring between each, until the mixture becomes thick, glossy, and slightly translucent.
Generously dust your work surface with cornstarch. Transfer the hot mochi dough onto the surface and let it cool slightly. Dust your hands and rolling pin, then gently roll or press the dough into a ¼-inch thick sheet.Cut out circles (about 3–4 inches in diameter) using a cookie cutter or the rim of a glass. Working quickly (but gently), place a frozen ball of tiramisu filling in the centre of each mochi circle. Pinch the edges together to seal, then roll into a ball.
Place seam-side down in a cupcake liner or muffin tray cup. Repeat with the remaining dough and filling. Lightly dust the tops with cocoa powder using a fine sieve.
Chill for 10–15 minutes before serving to allow the mochi to set and soften slightly around the creamy core.
