01 - Cream softened butter with brown sugar in small bowl until fluffy, about 2 minutes. Whisk in milk and vanilla until incorporated. Fold in heat-treated flour and salt until just combined. Gently stir in mini chocolate chips. Portion into 12 equal balls (approximately 2 teaspoons each), arrange on parchment-lined plate, and refrigerate until firm, at least 20 minutes.
02 - Combine butter and chopped dark chocolate in heatproof bowl. Microwave in 30-second intervals, stirring between each, or melt over double boiler until completely smooth. Set aside to cool slightly while preparing remaining ingredients.
03 - Beat granulated sugar, brown sugar, and eggs in large mixing bowl with hand mixer on medium-high speed for 2 minutes until mixture becomes pale and noticeably thicker. Blend in vanilla extract, then pour in melted chocolate mixture. Mix until fully uniform.
04 - Whisk flour, cocoa powder, baking powder, and salt in separate medium bowl until well distributed. Add dry mixture to wet ingredients and fold with spatula until thick, fudgy dough forms with no visible flour streaks. Refrigerate dough 10-15 minutes for easier handling.
05 - Position oven racks in center positions. Preheat oven to 350°F. Line two large baking sheets with parchment paper, ensuring complete coverage.
06 - Scoop approximately 2 tablespoons brownie dough per cookie. Flatten dough portion in palm of hand into disc shape. Place one chilled cookie dough ball in center, then wrap brownie dough around filling, pinching edges to seal completely. Repeat until all 12 cookies are formed.
07 - Arrange filled cookies on prepared baking sheets, spacing 2 inches apart. Bake 10-12 minutes until tops appear set with characteristic crackled surface, avoiding overbaking to preserve fudgy texture. Centers should still feel slightly soft when touched.
08 - Leave cookies on baking sheets 10 minutes to set. Transfer carefully to wire cooling rack and allow to cool completely before serving. This resting period allows centers to firm to proper consistency.