01 - In a small mixing bowl, cream the softened butter and light brown sugar together until light and fluffy, approximately 2 minutes. Whisk in the milk and vanilla extract until fully incorporated. Gradually stir in the heat-treated flour and salt until combined. Fold in the mini chocolate chips. Using a small cookie scoop or teaspoon, portion 12 equal balls (about 2 teaspoons each). Arrange on a parchment-lined plate and refrigerate until firm, at least 30 minutes.
02 - Place the unsalted butter and chopped dark chocolate in a heatproof bowl. Microwave in 30-second intervals, stirring between each, or melt over a double boiler until completely smooth. Allow the mixture to cool for 5-10 minutes until lukewarm to prevent cooking the eggs.
03 - In a large mixing bowl, whisk together the granulated sugar, light brown sugar, and eggs for 2 minutes until pale, thick, and doubled in volume. Add the vanilla extract and whisk to combine. Pour in the cooled melted chocolate mixture and fold gently until uniform in color.
04 - In a medium bowl, sift together the all-purpose flour, cocoa powder, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, stirring with a spatula until a thick, fudgy dough forms. Refrigerate for 10-15 minutes to firm slightly for easier handling.
05 - Preheat the oven to 350°F. Line two large baking sheets with parchment paper or silicone baking mats.
06 - Scoop approximately 2 tablespoons of brownie cookie dough per portion. Flatten the dough in your palm to create a disc. Place one chilled cookie dough ball in the center and carefully wrap the brownie dough around it, sealing completely so the filling is fully enclosed. Repeat for all 12 cookies.
07 - Arrange the cookies on the prepared baking sheets, leaving 2 inches of space between each. Bake for 10-12 minutes until the surfaces appear set with characteristic crackles, but the centers still feel slightly soft. Avoid overbaking to maintain fudgy texture.
08 - Let the cookies rest on the baking sheets for 10 minutes to firm. Transfer to a wire cooling rack and allow to cool completely before serving to achieve optimal texture.