Combine the glutinous flour, cornstarch, sugar, and milk in a mixing bowl. Make sure to whisk until they are combined thoroughly
Oil a shallow plate and pour the mixture onto the plate. Cover with a cling wrap and steam for about 20-25 minutes.
Proceed to "knead the mochi dough" step
To cook the mochi in the microwave:
Combine all ingredients in a microwave-safe bowl. Cover the bowl with a microwave-safe lid or cling wrap. Poke a few small holes in the wrap to allow steam to escape.
Microwave on high for 2 minutes. Remove and stir the batter with a silicone spatula. Continue microwaving in 1-minute intervals, stirring each time, until the mixture thickens and becomes sticky and translucent. Usually, this takes about 4–6 minutes total, depending on your microwave wattage.
Proceed to "knead the mochi dough" step
To knead the mochi dough:
Let it cool down for about 2-3 minutes and then scrape the dough onto an oiled silicone mat if you have one. Otherwise, parchment paper may work in a pinch
Use the silicone mat to help you "knead" the dough. The dough is still very warm and that's why the silicone mat comes in handy here. If you use parchment paper, you may want to wear a kitchen mitten so it won't be too hot for your hands
Keep kneading until the dough is really soft and pliable. Halfway through kneading, add the cocoa powder and continue to knead until it is evenly distributed.
This kneading process is what gives the mochi its amazing pliable stretchy texture
You can also put the dough in a standmixer bowl and use the flat beater to beat the dough until it is soft and pliable
Cover with a cling wrap and let it cool down when you prepare the cookie dough
Prepare the cookie dough
Combine the all-purpose flour, matcha powder, baking powder, and 1/4 tsp salt
About 30 minutes to 1 hour before you plan to bake, get the butter and egg out of the fridge and let them come to room temperature
Beat the softened butter and brown sugar vigorously until they are creamy. You don't have to use a mixer for this. Add a room-temperature egg and continue to mix again until combined. If it appears curdled, don't worry, it will come together once we add the dry mixture
Sift in the flour mixture and use a spatula to fold until everything is well-combined.
Flatten the dough into a disc, wrap with a cling wrap, and chill in the fridge for at least 30 minutes
Assembling:
Line two large baking sheets with parchment paper. Preheat the oven to 350 F (180 C) for conventional oven, 330 F (165 C) for convection oven
Portion the mochi dough into 18 portions (about 20 grams each) and round it up into a ball for easier wrapping. Portion the cookie dough into 18 portions (about 25 grams each)
Flatten the cookie dough with your palm, probably about 3-4 inches in diameter, roughly. Place one mochi dough in the center and wrap the cookie dough around the mochi dough to enclose.
The assembled cookie dough is kinda chubby and tall at this point. Flatten it slightly and press a few white chocolate chips on top
Continue with the rest of the cookie dough and mochi dough. Place the assembled cookie dough on the prepared baking sheet, about 2-inches apart. Flatten the cookie dough again a little bit. The cookies don't spread much
Baking:
I recommend baking one tray at a time. Place the baking pan in the middle rack of the oven and bake for 12-13 minutes. The edges should be just lightly browned and the center should still be soft. DO NOT overbake the cookies. They may appear soft coming out of the oven
Cool down:
Let the cookies cool down in the pan for 5 minutes and then transfer to the cooling rack to cool down completely
Storage:
Once the cookies have cooled down, transfer to an air-tight container. The mochi stays soft for 2-3 days at room temperature.