01 - Cream butter and brown sugar in small bowl until light and fluffy. Mix in milk and vanilla. Stir in heat-treated flour and salt until combined. Fold in mini chocolate chips. Scoop 12 small balls (2 teaspoons each), place on parchment-lined plate, and refrigerate.
02 - Melt butter and dark chocolate together in heatproof bowl using microwave in 30-second bursts or double boiler. Stir until smooth and let cool slightly.
03 - Beat granulated sugar, brown sugar, and eggs in large mixing bowl for 2 minutes until pale and creamy. Add vanilla extract and melted chocolate mixture, mixing until combined.
04 - Whisk together flour, cocoa powder, baking powder, and salt in separate bowl. Add to wet ingredients and mix until thick dough forms. Chill dough for 10-15 minutes for easier handling.
05 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
06 - Scoop 2 tablespoons brownie cookie dough per cookie. Flatten in palm. Place chilled cookie dough ball in center and wrap brownie dough around it, sealing well. Repeat for all cookies.
07 - Place cookies on prepared trays spaced apart. Bake 10-12 minutes until tops are just set with crackles. Do not overbake for optimum fudginess.
08 - Allow cookies to cool on tray for 10 minutes before transferring to rack to cool completely.