01 - Cream softened butter and brown sugar in a small bowl until light and fluffy, approximately 2 minutes. Whisk in milk and vanilla extract until smooth. Fold in heat-treated flour and salt until fully incorporated. Gently stir in mini chocolate chips. Portion into 12 equal balls, about 2 teaspoons each. Arrange on parchment-lined plate and refrigerate until firm, at least 20 minutes.
02 - Combine butter and chopped dark chocolate in a heatproof bowl. Microwave in 30-second intervals, stirring between each, until completely smooth and melted. Alternatively, melt over a double boiler, stirring constantly. Allow mixture to cool for 5 minutes before proceeding.
03 - Beat granulated sugar, brown sugar, and eggs in a large mixing bowl for 2 minutes until pale and creamy. Incorporate vanilla extract. Gradually pour in melted chocolate mixture, whisking continuously until fully combined and glossy.
04 - Whisk together flour, cocoa powder, baking powder, and salt in a separate bowl. Gradually add dry ingredients to wet mixture, folding gently until thick, fudgy dough forms. Refrigerate dough for 10-15 minutes to firm slightly for easier handling.
05 - Preheat oven to 350°F. Line two large baking sheets with parchment paper.
06 - Scoop approximately 2 tablespoons of brownie dough per cookie. Flatten dough in palm of hand to create a disc. Place one chilled cookie dough ball in center. Gently wrap brownie dough around filling, sealing edges completely to encase. Repeat with remaining dough and filling.
07 - Arrange cookies on prepared baking sheets, spacing 2 inches apart. Bake for 10-12 minutes until tops are set with characteristic crackled appearance. Centers should still appear slightly soft. Avoid overbaking to maintain fudgy texture.
08 - Let cookies rest on baking sheets for 10 minutes to set. Transfer to wire rack and cool completely before serving or storing.