01 - Cream softened butter and brown sugar in small bowl until fluffy, about 2 minutes. Blend in milk and vanilla. Stir in heat-treated flour and salt until incorporated. Fold in mini chocolate chips. Roll dough into 12 equal balls (about 2 tsp each), arrange on parchment-lined plate, and refrigerate until firm.
02 - Combine butter and chopped dark chocolate in heatproof bowl. Microwave in 30-second intervals, stirring between each, or melt over double boiler. Stir until completely smooth and glossy. Allow mixture to cool for 5 minutes.
03 - Whisk granulated sugar, brown sugar, and eggs in large mixing bowl for 2 minutes until pale and thickened. Add vanilla extract, then gradually pour in cooled chocolate mixture, stirring constantly until fully blended.
04 - Sift flour, cocoa powder, baking powder, and salt into separate bowl. Add dry ingredients to wet mixture, folding gently until thick, fudgy dough forms. Refrigerate dough for 10-15 minutes to firm slightly for easier handling.
05 - Set oven to 350°F. Line two large baking sheets with parchment paper, ensuring complete coverage to prevent sticking.
06 - Portion approximately 2 tablespoons brownie dough per cookie. Flatten dough in palm of hand. Place one chilled cookie dough ball in center. Encase filling completely with brownie dough, pinching edges to seal. Repeat with remaining dough.
07 - Arrange stuffed cookies on prepared baking sheets, leaving 2 inches between each. Bake for 10-12 minutes until tops appear set with characteristic crackled surface. Centers should still feel slightly soft—avoid overbaking for optimal fudgy texture.
08 - Leave cookies on baking sheets for 10 minutes to set. Transfer to wire rack and cool completely before serving to allow centers to firm.