Make the ice cream. Whip the heavy cream until it forms soft peaks.
Soften the bean paste with a whisk, add to the whipped cream, and mix well. Freeze for at least 2 hours, or until it's half-frozen.
Spread a generous amount of cornstarch on a large cutting board.
Put the shiratamako in a microwave-safe bowl, and stir in water, a little at a time. Add sugar, and continue mixing.
Lightly cover with plastic wrap and microwave (600w) for 2 minutes. Take it out, mix, and heat again for 1 more minute.
If it's light and soft, it's ready. If not, add 1 teaspoon of water and heat for 20 seconds, or until shiny.
Roll the dough out on the cutting board covered with cornstarch, to a thickness of 2~3mm. It's hot at first, but it should be easy to handle, so take your time rolling it. After it cools, cut into pieces, and discard the uneven edges.
Layer each piece between plastic wrap, and freeze. That way, you can just add ice cream whenever you feel like eating.
Lay the dough horizontally. (The photo was a bit unclear, so I drew lines.) Put a spoonful of ice cream in the middle. Dip your finger in water, and wet the part along the lines. Fold over from left to right, and seal the edges firmly.
Line up in a flat container, cover tightly with plastic wrap and freeze for 3~4 hours. If you put them in a ziplock bag after they're frozen, they'll stay soft.
You can make all kinds of variations by using different flavors of ice cream.
Story Behind this Recipe
I created this recipe while I was in the middle of making Yukimi Daifuku-like ice cream. I asked a friend to taste it, and she gave me the thumbs up, so I decided to upload the recipe.
If it's completely frozen, the ice-cold mochi melts in your mouth. If it's partially frozen, the mochi and bean paste ice cream come together in perfect harmony. If it's melted, it's basically mochi coated with bean paste cream, which is also great. Tip: Try to roll out the dough thinly in Step 7. Try as hard as you can not to expose it to air while it's freezing.