01 - Cream the softened butter and brown sugar in a small bowl until light and fluffy. Blend in milk and vanilla extract. Stir in heat-treated flour and salt until fully combined, then fold in mini chocolate chips. Form 12 small balls (approximately 2 tsp each), arrange on parchment-lined plate, and refrigerate until firm.
02 - Combine butter and chopped dark chocolate in a heatproof bowl. Microwave in 30-second intervals, stirring between each, or melt over double boiler until smooth. Allow mixture to cool slightly before proceeding.
03 - Beat granulated sugar, brown sugar, and eggs in a large mixing bowl for approximately 2 minutes until mixture becomes pale and creamy. Incorporate vanilla extract and melted chocolate mixture, blending until fully combined.
04 - Whisk together flour, cocoa powder, baking powder, and salt in a separate bowl. Gradually add dry ingredients to wet mixture, stirring until thick dough forms. Refrigerate dough for 10-15 minutes to facilitate handling.
05 - Preheat oven to 350°F. Line two baking sheets with parchment paper and set aside.
06 - Portion approximately 2 tbsp of brownie cookie dough per cookie. Flatten each portion in palm. Place one chilled cookie dough ball in center and enclose completely with brownie dough, ensuring proper seal. Repeat for remaining cookies.
07 - Arrange stuffed cookies on prepared baking trays with adequate spacing. Bake for 10-12 minutes until surfaces appear set with characteristic crackling. Avoid overbaking to preserve fudgy texture.
08 - Let cookies rest on baking sheets for 10 minutes to set. Transfer to wire rack and cool completely before serving.