01 - Cream 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. Gently stir 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. Microwave in 30-second intervals, stirring between each, or melt over a double boiler. Stir until completely smooth and set aside to cool slightly.
03 - Beat granulated sugar, brown sugar, and eggs in a large mixing bowl for 2 minutes until pale and creamy. Whisk in vanilla extract followed by the cooled chocolate mixture until incorporated.
04 - Whisk together flour, cocoa powder, baking powder, and salt in a separate bowl. Add dry ingredients to wet mixture and stir until thick dough forms. Refrigerate for 10-15 minutes to firm for easier handling.
05 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
06 - Scoop approximately 2 tablespoons of brownie dough per cookie. Flatten in palm of hand. Place one chilled cookie dough ball in center and wrap brownie dough around, sealing completely. Repeat with remaining dough.
07 - Arrange cookies on prepared baking sheets, leaving space between each. Bake for 10-12 minutes until surfaces are set with crackled appearance. Avoid overbaking to maintain fudgy texture.
08 - Let cookies rest on baking sheets for 10 minutes, then transfer to wire rack to cool completely.