Matcha Mochi Cookies

1.In a medium-sized bowl, whisk together glutinous rice flour, 1/4 cup sugar, and 1 tablespoon matcha powder. Add coconut milk and water to the dry mixture. Whisk until smooth. If there are clumps, push the mixture through a fine-meshed sieve.
2.Cover bowl with plastic wrap. Put the covered bowl into a steamer basket and steamed for 20 minutes. Halfway cooking, stir with a wet rubber spatula and cover to finish cooking. The mochi should change from opaque to almost translucent. Allow mochi to cool completely. Place potato starch on a plate and divide dough into 10 portions for the cookies. Use more potato starch if needed to prevent mochi from sticking.
3.In the bowl of a stand mixer fitted with a paddle attachment, cream the butter, brown sugar, and 1/2 cup sugar on medium speed until they are smooth, about 1 minute. Add the vanilla and egg to the butter mixture and mix on medium speed until they are fully incorporated, scraping down the sides of the bowl as needed.
4.In a medium bowl, whisk together the flour, 1 tablespoon matcha powder, salt, baking powder, and baking soda. With the mixer on low speed, add the flour mixture in two batches to the butter mixture, and mix until the ingredients are just combined, about 1 minute.
5.Separate the chopped chocolates into two portions; reserve the larger pieces to top each cookie dough ball before baking. Add one portion of the chocolates to the bowl, and mix until they are just incorporated with the dough, about 30 seconds. Set second portion of chocolates aside. Preheat the oven to 350°F. Line a large baking sheet with parchment paper.
6.With a standard ice cream scoop, form even balls of dough. Using your thumb, create a deep indent in the cookie dough ball. Fill each cavity with a piece of cooled matcha mochi. Seal the doughball. Place filled dough balls about 2 inches apart on the baking sheet. Place several pieces of reserved chocolate onto each dough ball.
7.Bake the cookies for 12 to 14 minutes, or until the cookies are browned and caramelized along the edges and the centers are just set. If you would like your cookies to have the crinkled edges, give the pan a few taps on the oven rack before you take them out of the oven.
8.Allow the cookies to cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely. Mochi-filled cookies are best once they have cooled completely.