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