Place the dry ingredients (joshinko, shiratamako, and sugar) in a bowl and mix together.
Add the hot water gradually while checking the firmness of the dough. It's hot, so use a spatula or chopsticks to mix first.
Once it has come together, knead it thoroughly with your hands.
Tear the dough into pieces and place in a steamer basket, making sure they do not overlap so that it steams evenly.
Steam over medium heat for about 10 minutes.
Once the steaming is done, transfer to a bowl, crushing with a rolling pin or pestle while it is still hot.
After it has been crushed to a certain extent, knead it with your hands.
If you quickly dip it in ice water halfway through to cool it down, it will become glossy.
Remove from the iced water, knead again, roll out into a log and cut into 6 equal pieces.
Roll into balls, lightly crush with your hands, then use a rolling pin to stretch out the dough up and down to an oval shape and match the size of the oak leaves.
Place an anko azuki bean paste ball in the center and sandwich it with the dough.
Place the smooth side of the oak leaves inwards to wrap the dough. Seal the center of the seam tightly.
Arrange them on a steamer basket and steam for about 5 minutes. This last step is an optional step, but it helps bring out the aroma of the oak leaves.
Notes
*1 If you are making anko sweet bean paste from scratch see this recipe here and you need to make it the night before. *2 and *3 If you can't access these specific flours, see the substitutes in the above post. *4 If you can't get oak leaves, see the substitutes in the above post. You can also have this mochi without the leaves since oak leaves are not edible and are just for aroma and appearance.