Rinse oak leaves in running water. If the oak leaves you purchased are the dried kind, boil for 10 minutes and then soak in water. Dry with paper towel.
Wet your hands with water and scoop 1 heaping spoon of red bean paste (25 g) onto your hand. Shape it into a ball. Repeat until you have 10 balls.
In a large bowl combine 7 oz (200 g) Joshinko, 2 Tbsp potato starch, and 2 Tbsp sugar and mix all together with a whisk.
Add 260 ml (1 cup + 4 tsp)water and mix well with the whisk.
Loosely cover with plastic wrap and microwave for 4 minutes (1100 watt microwave). Please adjust the cook time according to your microwave’s power.
Loosen the mochi with spatula. Soak the wooden pestle (or anything that you can pound with) in water and start pounding.
Mochi will get sticker and shinier by pounding. Make sure to soak the pestle in water between poundings. It will help mochi become shiny and glossy.
Once mochi is cool enough for you to handle, transfer the mochi on moist working surface. Start kneading with your wet hands until glossy, elastic, and smooth.
Cut the mochi in half then cut into 5 pieces.
Shape each piece into a round ball then cover the mochi under a damp towel to prevent from drying. With a rolling pin, roll mochi ball into an oval shape.
Place a red bean paste ball in the center of the mochi and fold in half to seal the edge. Make sure your fingers are clean. If your finger has red bean paste on it or if the red bean paste overfills out of the seam line, the sides won’t stick each other and it’ll become very hard to seal. What I do is pull mochi away from red bean paste and seal tightly first, then reshape the mochi to make them look nice.
Wrap mochi in an oak leaf. The back side of the leaf (veins are more visible) should be faceing outside for red bean filling (Read the post for more details).
It's best to consume on the same day as the mochi will get hard, especially if you put it in the refrigerator. Store it in a cool place.