In a large mixing bowl, combine the sweet rice flour and sugar. Stir them until they are well mixed.

Gradually add in the water, stirring continuously until the mixture becomes a smooth batter with no lumps.

Transfer the batter to a microwave-safe dish. Cover it with plastic wrap and microwave on high for about 1 minute. Carefully remove it, stir, and then microwave for an additional minute. Repeat this process one more time, or until the dough becomes sticky and slightly translucent.

Dust a clean surface with cornstarch to prevent sticking. Transfer the mochi dough onto the cornstarch-dusted surface and allow it to cool for a few minutes.

Once cool enough to handle, divide the dough into small pieces (about 2 tablespoons each). Flatten each piece in your hand and place a spoonful of Nutella in the center.

Wrap the mochi around the filling, pinching the edges to seal it completely, then gently roll it into a ball.

Roll the finished mochi in cornstarch to prevent them from sticking together and set aside to cool completely.