01 - Cream 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 (about 2 teaspoons 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 or melt over double boiler, stirring until smooth. Allow mixture to cool slightly before proceeding.
03 - Beat granulated sugar, brown sugar, and eggs in a large bowl for approximately 2 minutes until mixture becomes pale and creamy. Incorporate vanilla extract and cooled melted chocolate, mixing until thoroughly combined.
04 - Whisk together flour, cocoa powder, baking powder, and salt in a separate bowl. Add dry mixture to wet ingredients and mix until thick, uniform dough forms. Refrigerate dough for 10-15 minutes to improve handling.
05 - Set oven to 350°F. Line two baking sheets with parchment paper and set aside.
06 - Portion approximately 2 tablespoons 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, sealing edges thoroughly. Repeat for remaining portions.
07 - Arrange assembled cookies on prepared baking trays with adequate spacing. Bake for 10-12 minutes until surfaces are just set with characteristic crackled appearance. Avoid overbaking to preserve fudgy texture.
08 - Let cookies rest on baking sheets for 10 minutes before transferring to wire rack. Allow to cool completely before serving.