In a large mixing bowl, combine the sweet rice flour and sugar. Stir until well mixed and no lumps remain.

Gradually add in the water, stirring continuously until the mixture becomes smooth, thick, and glossy.

Transfer the batter to a microwave-safe dish and cover it with plastic wrap. Microwave on high for about 1 minute. Carefully remove, stir, and microwave for another minute. Repeat this process for a total of three times or until the dough is sticky and slightly translucent.

Dust a clean surface with cornstarch to prevent sticking. Transfer the warm mochi dough onto the dusted surface and let it cool until manageable but still warm.

Once cool enough to handle, divide the dough into small pieces (about 2 tablespoons each). Flatten each piece in your hand, place a generous spoonful of Nutella in the center, then wrap the mochi around the filling, pinching the edges to seal it completely.

Gently roll into a ball and then roll the finished mochi in cornstarch to prevent them from sticking together.

Set aside to cool completely, allowing the texture to firm up.