1. In a small saucepan, combine all the ingredients for the mochi. Set the heat to medium and whisk quickly. When it starts to thicken and pull away from the sides, switch to stirring with a spatula to make it easier.
2. Stir until the mixture comes away from the saucepan and forms a pliable dough. Dust your work surface with the sweet sticky rice flour (or cornstarch) and roll the dough into a log. Set aside to cool. Cut small pieces, about 2-3 cm, and flatten them with your hands into a round wrapper. Place your desired filling in the center. Close it up by gathering the edges around. Seal well and turn the mochi upside down. If you prefer, you can use a small bowl to help shape the mochi.
3. Place the wrapper inside the bowl (if using), put 1 teaspoon of filling in the middle, gather the wrapper around it, and seal. Repeat the process with the remaining ingredients. If you are storing the dough, cover it with plastic wrap. Make the mochi and serve immediately.
