01 - In a small bowl, cream the softened butter and brown sugar together until light and fluffy, about 2 minutes. Mix in the milk and vanilla extract until smooth. Stir in the heat-treated flour and salt until just combined, then fold in the mini chocolate chips. Scoop 12 small balls (approximately 2 teaspoons each), place them on a parchment-lined plate, and refrigerate while preparing the brownie cookie dough.
02 - In a heatproof bowl, combine the butter and chopped dark chocolate. Melt together in the microwave in 30-second bursts, stirring between each interval, or over a double boiler. Stir until completely smooth and let cool slightly for 5 minutes.
03 - In a large mixing bowl, beat together the granulated sugar, brown sugar, and eggs using a hand mixer for about 2 minutes until pale and creamy. Add the vanilla extract and the cooled melted chocolate mixture, mixing until fully incorporated.
04 - In a separate medium bowl, whisk together the flour, cocoa powder, baking powder, and salt until well blended. Add the dry ingredients to the wet mixture and fold together until a thick, fudgy dough forms. Refrigerate the dough for 10-15 minutes to firm slightly for easier handling.
05 - Preheat oven to 350°F. Line two large baking sheets with parchment paper and set aside.
06 - Scoop approximately 2 tablespoons of brownie cookie dough per cookie. Flatten each portion in the palm of your hand into a disc. Place one chilled cookie dough ball in the center and carefully wrap the brownie dough around it, pinching the seams to seal completely. Repeat for all 12 cookies.
07 - Arrange the stuffed cookies on the prepared baking sheets, spacing 2 inches apart. Bake for 10-12 minutes until the tops are just set with a crackled appearance. The centers should still be slightly soft—do not overbake for optimal fudgy texture.
08 - Allow the cookies to cool on the baking sheets for 10 minutes to set. Transfer to a wire rack and let cool completely before serving, about 20 additional minutes.