In a large mixing bowl, combine the sweet rice flour and sugar. Stir until the mixture is homogeneous.

Gradually add in the water, stirring continuously until you achieve a smooth batter with no lumps. The consistency should be liquid-like.

Transfer the batter to a microwave-safe dish and cover it with plastic wrap. Microwave on high for about 1 minute, then stir the mixture. Repeat this process for an additional minute, and continue microwaving in 30-second intervals until the dough is sticky and slightly translucent, about 3-4 minutes total.

Dust a clean surface generously with cornstarch. Carefully transfer the mochi dough onto the dusted surface and allow it to cool for a few minutes until you can handle it.

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

Carefully wrap the mochi around the filling, pinching the edges to seal it, then roll it into a ball.

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