01 - Cream softened butter and brown sugar in a small bowl until light and fluffy. Whisk in milk and vanilla extract. Fold in heat-treated flour and salt until fully combined, then gently mix in mini chocolate chips. Form 12 small balls approximately 2 teaspoons each, arrange on parchment-lined plate, and refrigerate until firm.
02 - Combine butter and chopped dark chocolate in a heatproof bowl. Melt using microwave in 30-second intervals, stirring between each, or place over a double boiler. Stir until completely smooth and glossy, then set aside to cool slightly.
03 - Beat granulated sugar, brown sugar, and eggs in a large mixing bowl for approximately 2 minutes until mixture becomes pale and creamy. Pour in vanilla extract and cooled melted chocolate mixture, whisking until thoroughly incorporated.
04 - Whisk together flour, cocoa powder, baking powder, and salt in a separate bowl. Gradually add dry ingredients to wet mixture, stirring until a thick, fudgy dough forms. Refrigerate for 10-15 minutes to firm slightly for easier handling.
05 - Set oven to 350°F. Line two baking sheets with parchment paper.
06 - Portion approximately 2 tablespoons of brownie cookie dough per cookie. Flatten each portion in palm of hand. Place one chilled cookie dough ball in center and enclose completely with brownie dough, pinching edges to seal tightly. Repeat with remaining dough.
07 - Arrange cookies on prepared baking trays, leaving 2 inches between each. Bake for 10-12 minutes until surfaces appear set with characteristic crackling. Remove promptly for optimal fudgy texture.
08 - Let cookies rest on baking sheets for 10 minutes to set. Transfer to wire rack to cool completely.