01 - Cream softened butter and brown sugar in a small bowl until pale and fluffy, approximately 2 minutes. Whisk in milk and vanilla extract until smooth. Fold in heat-treated flour and salt until fully incorporated, then gently mix in mini chocolate chips. Portion dough into 12 equal balls (about 2 tsp 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. Melt using microwave in 30-second intervals, stirring between each, or place over simmering water in a double boiler. Stir until completely smooth and glossy. Allow mixture to cool for 5 minutes before proceeding.
03 - In a large mixing bowl, whisk granulated sugar, brown sugar, and eggs on medium-high speed for 2 minutes until mixture becomes thick and pale. Add vanilla extract and gradually pour in melted chocolate mixture, whisking continuously until fully combined.
04 - Sift together flour, cocoa powder, baking powder, and salt in a medium bowl. Gradually add dry ingredients to wet mixture, folding with a spatula just until a thick, fudgy dough forms. Refrigerate for 10-15 minutes to firm slightly for easier handling.
05 - Position oven racks in upper and lower thirds. Preheat oven to 350°F. Line two large baking sheets with parchment paper or silicone baking mats.
06 - Scoop approximately 2 tbsp of brownie dough per cookie. Flatten dough between palms to create a disc. Place one chilled cookie dough ball in center and carefully wrap brownie dough around it, sealing edges completely. Repeat with remaining dough and filling balls.
07 - Arrange cookies on prepared baking sheets, leaving 2 inches between each. Bake for 10-12 minutes until tops appear set with characteristic crackled surface. Centers should still feel slightly soft to touch. Remove from oven immediately.
08 - Let cookies rest on baking sheets for 10 minutes to set. Transfer to wire cooling rack and allow to cool completely before serving or storing.