1. Start with the Nutella filling. Use a teaspoon and place about 20-25 portions of Nutella on the prepared baking sheet. Place them gently in the freezer for 20 to 30 minutes. It will be harder and easy to use it later.
2. Meanwhile, prepare mochi mixture. In a microwave safe glass bowl, combine the flour and sugar. Stir to combine. Add the water slowly and mix with an egg beater until smooth. Cover the bowl with cling wrap.
3. Microwave the mixture for 1 minute. Then, stir the dough with a wet spatula. put it back in the microwave again for another minute. If gets sticky then put it back in the microwave for another 30 seconds (not 1 min!). Repeat it until the mixture is very thick and translucent.
4. Transfer the mochi dough on to the surface floured with katakuriko/potato starch, it is very hot so use a spatula. Equally cut the mochi into 20-25 pieces. Don't forget to coat your hand with katakuriko/potato starch.
5. With your hands, take a piece of the ball and flatten it into a circle (about 4 to 5 cm). Then put one piece of nutella ball from the freezer (one at a time as the nutella ball will melt quickly) into the middle of the mochi, pull the edges together to cover the nutella ball, and press together to seal. Smoothen the mochi by rolling it on the dusted board.
Keep in an airtight container.
For a cream filling instead of nutella
i. Pour the cream in a medium size bowl, and add the sugar.
ii. Whisk just until the cream reaches stiff peaks. Spoon about 1/2 tsp and place about 20-25 portions of jam on the prepared baking sheet and spoon the whipped cream on it gently (See photo).
iii. Place them gently in the freezer for 30 to 40 minutes. It will be harder and easy to use it later.
In this case keep your mochi in an airtight container and store them in the fridge.