01 - Cream softened butter and brown sugar in small bowl until fluffy, about 2 minutes. Whisk in milk and vanilla. Fold in heat-treated flour and salt until just combined. Gently stir in mini chocolate chips. Scoop 12 portions (2 teaspoons each) onto parchment-lined plate. Refrigerate until firm, about 20 minutes.
02 - Place butter and chopped dark chocolate in heatproof bowl. Microwave in 30-second intervals, stirring after each, until completely smooth. Alternatively, melt over double boiler, stirring constantly. Let cool 5 minutes.
03 - Beat granulated sugar, brown sugar, and eggs in large bowl with electric mixer on medium-high speed until pale and thickened, about 2 minutes. Add vanilla and melted chocolate mixture. Mix on low speed until fully incorporated.
04 - Whisk together flour, cocoa powder, baking powder, and salt in medium bowl. Add dry ingredients to wet mixture. Mix on low speed just until thick dough forms. Do not overmix. Refrigerate dough 15 minutes for easier handling.
05 - Preheat oven to 350°F. Line 2 large baking sheets with parchment paper. Set aside.
06 - Scoop 2 tablespoons brownie dough per cookie. Flatten dough in palm. Place 1 chilled cookie dough ball in center. Wrap brownie dough completely around filling, sealing edges tightly. Repeat with remaining dough and filling.
07 - Place cookies 2 inches apart on prepared baking sheets. Bake 10-12 minutes until tops are set with crackled appearance. Centers should still appear slightly underbaked. Rotate pans halfway through baking.
08 - Let cookies cool on baking sheets 10 minutes to set. Transfer to wire rack to cool completely, about 30 additional minutes. Centers will firm as they cool.