01 - In a small bowl, cream the 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 using the microwave in 30-second bursts, stirring between intervals, or set over a double boiler. Stir until completely smooth and glossy, then set aside to cool slightly.
03 - In a large mixing bowl, beat together the granulated sugar, brown sugar, and eggs using a hand mixer or whisk for approximately 2 minutes until the mixture is pale in color and creamy in texture. 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 evenly distributed. Add the dry ingredients to the wet mixture and mix with a spatula or wooden spoon until a thick, fudgy dough forms. Refrigerate the dough for 10-15 minutes to firm up for easier handling.
05 - Preheat the oven to 350°F. Line two large baking sheets with parchment paper and set aside.
06 - Scoop approximately 2 tablespoons of brownie cookie dough for each cookie. Flatten the dough piece in the palm of your hand to create a disc. Place one chilled cookie dough ball in the center and carefully wrap the brownie dough around it, sealing the edges completely to enclose the filling. Repeat this process for all 12 cookies.
07 - Place the formed cookies onto the prepared baking trays, spacing them 2 inches apart to allow for spreading. Bake for 10-12 minutes, or until the tops are just set with characteristic crackles appearing on the surface. The centers should still appear slightly soft for optimal fudginess.
08 - Allow the cookies to cool on the baking trays for 10 minutes to set their structure. Carefully transfer them to a wire cooling rack to cool completely before serving.