For the bean paste: In a pot, combine the red beans and sugar. Cook over medium-low heat for 10 minutes until the sugar is dissolved, adding water if needed. When the beans become soft, mash into a paste.
Spread the mixture out onto a quarter sheet tray and cover with plastic wrap. Put in the freezer or fridge to cool quickly.
For the dough: In a heat-safe bowl, combine the sweet rice flour with the water and stir with a wooden spoon until crumbly.
Put the bowl over a double boiler and cover with a tea towel to steam for 15 to 20 minutes.
Move the steamed dough to a saucepan and add the sugar and salt, mixing for 10 minutes over medium to medium-low heat until the dough pulls away from the side and starts to ribbon. A little crust will start to form on the bottom.
Dust a nonstick baking mat with cornstarch and knead dough, using more cornstarch if needed.
Roll into a snake, about 1 inch in diameter, and cut into 2-inch pieces. Shape cut pieces into balls, keeping all unused mochi covered.
Flatten each ball into a circle, about 2 1/2-inches in diameter. Put on the cornstarch-dusted silicone baking mat, brushing off excess cornstarch before adding the filling.
Put a scoop of cooled red bean paste in the center of the mochi dough and wrap up to encase the filling, gently pinching the seams to seal.
Transfer completed mochi to the fridge to cool. Mochi is best the day it’s made.