In a large bowl add the softened butter, salt, and brown sugar. Use a hand mixer to combine until pale and fluffy, for about 2 minutes.
Add in the egg, then mix again to combine into a smooth paste.
Sift in the flour and baking soda. Use a hand mixer to combine, only mix until the dough is just about combined. Do not over-mix! The cookie dough will be very soft, but do not add extra flour.
Cover the dough with plastic wrap and refrigerate for at least 1-2 hours. Chilling will make the dough harder and easier to handle.
Once hardened, divide the dough into 12 equal parts using an ice cream scoop.
Roll the pieces into small balls. Place them on a large plate, cover with plastic wrap and refrigerate until ready to use.
Mochi filling
In a small glass bowl, add the mochiko flour, sugar, and warm milk. Mix well to combine into a lump-free paste.
Cover the bowl with plastic wrap, then poke a few holes in the film with a toothpick. Microwave for 30-45 seconds.
Remove plastic wrap from bowl, then use a spatula to mix the paste and break up larger chunks. Cover the bowl with the pieced plastic wrap, then microwave again for 45 seconds to 1 minute.
Remove foil, then mix well with a spatula. At this point, the mochi dough should be very sticky. Keep mixing until the dough looks smooth.
Add butter into the cooked mochi and mix well until the butter is fully incorporated. Cover and let cool down completely.
Dust a plate with corn starch and place the mochi dough onto it. Dust the top with more corn starch, then press down on the dough to flatten it. Divide the dough into 12 equal pieces the size of a coin.
Assembling the Mochi Cookies
Pre-heat oven to 350°F (180°C) and line two large baking sheets with parchment paper.
Flatten the mochi piece with the palm, then place a piece of chocolate in the middle.
Wrap the sides of the mochi dough over the chocolate, encasing it inside. Roll in palm to make it round.
Flatten each piece of cookie dough into a disc. Place a chocolate mochi in the middle of the cookie dough. Wrap the dough around to encase the mochi completely.
Roll the cookie to make it round. Repeat for the remaining cookies. Place all cookies on the baking sheets, leaving at least 2-3 inches between them.
Add chocolate chips on top of all the cookies, pressing down to make it stick to the dough.
Bake for 10-12 minutes or until golden-brown in colour.
Let the cookies cool down on the tray for 10 minutes, then carefully transfer them to a cooling rack. Enjoy whilst still warm for the best texture!
Notes
I recommend using a kitchen scale in grams for more accuracy. The cups used for the conversion are standard US customary cups (1 cup flour = 136g). There are many different types of cups across the globe, which is why I strongly recommend using grams instead.
Mochiko flour can be replaced with any other type of sweet glutinous rice flour.
Nutritional value is estimative and it's calculated per serving (this recipe makes 12 cookies).
Serve within a day for the best texture.
Store in an airtight container at room temperature for maximum 2 days.