Start with the mochi filling. In a microwave-safe bowl, whisk together the sweet rice flour, sugar, and milk until smooth. Microwave for 1 minute, stir well, then microwave for another 30 seconds. Stir in the melted butter until the mixture forms a smooth, stretchy dough.
Let the mochi cool slightly until you can handle it. Divide it into 12 equal pieces and roll each into a small ball. Place them on a parchment-lined plate and freeze for at least 30 minutes until firm.
In a medium bowl, whisk together the almond flour, cocoa powder, tapioca starch, baking soda, and salt. Set aside.
In a large bowl, beat the softened butter and brown sugar together until fluffy, about 3 minutes. Add the egg and vanilla extract and beat until well combined. Gradually mix in the dry ingredients until a thick dough forms, then fold in the chocolate chips.
Scoop about 2 tablespoons of cookie dough, flatten it in your palm, and place a frozen mochi ball in the center. Wrap the cookie dough completely around the mochi, sealing it shut and rolling it into a smooth ball. Repeat with the remaining dough and mochi. Place them on a parchment-lined baking sheet and refrigerate for 20 minutes.
Preheat your oven to 350F (175C). Bake the cookies for 11 to 13 minutes until the outsides are set but the centers still look soft. Do not overbake because the mochi centers need to stay chewy and stretchy.
Let the cookies cool on the baking sheet for 10 minutes before transferring to a wire rack. Serve warm for the best mochi pull and gooey center.