Prepare the ingredients. Peel the skins from the grapes.
Mix the shiratamako, sugar, and water in a heat-proof dish.
Microwave for 1 minute and 30 seconds and mix (this is how it should look after microwaving).
Microwave for another minute and a half, and mix (this is how it should look after the second round).
Repeat by microwaving for another minute and a half (this is how it should look after the final round). This is your gyuhi for the outer dough.
Divide the gyuhi into 12 equal portions on a working surface dusted with mochiko.
Flatten the pieces of gyuhi, spread on a layer of tsubu-an, place a grape on top, then wrap it up. (Repeat for the remaining dumplings)
Reshape the dumplings, then place them in an aluminium baking cup. They fit perfectly into a No. 6 baking cup.