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 (about 2 teaspoons each), place on parchment-lined plate, and refrigerate while making brownie cookies.
02 - Melt butter and dark chocolate together in heatproof bowl using microwave in 30-second bursts or over 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 flour, cocoa powder, baking powder, and salt in separate bowl. Add to wet ingredients and mix until thick dough forms. Chill dough 10-15 minutes for easier handling.
05 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
06 - Scoop about 2 tablespoons brownie cookie dough per cookie. Flatten each piece 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, spacing 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 10 minutes before transferring to rack to cool completely.