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 fully incorporated. Fold in heat-treated flour and salt until just combined, then gently mix in mini chocolate chips. Portion dough into 12 equal balls (about 2 teaspoons each), arrange on parchment-lined plate, and refrigerate until firm, at least 30 minutes.
02 - Combine butter and chopped dark chocolate in a heatproof bowl. Microwave in 30-second intervals, stirring thoroughly between each burst, until completely smooth and glossy. Alternatively, melt over a double boiler, stirring constantly. Allow mixture to cool for 5-10 minutes before proceeding.
03 - Using a hand mixer or stand mixer, beat granulated sugar, brown sugar, and eggs on medium-high speed for 2 minutes until pale, thick, and doubled in volume. Add vanilla extract and cooled chocolate mixture, mixing on low speed until fully incorporated. Scrape down bowl sides as needed.
04 - In a separate medium bowl, whisk together flour, cocoa powder, baking powder, and salt until well blended. Gradually add dry ingredients to wet mixture, mixing on low speed just until thick, fudgy dough forms. Avoid overmixing. Refrigerate dough for 10-15 minutes for easier handling.
05 - Preheat oven to 350°F. Line two large baking sheets with parchment paper, ensuring complete coverage.
06 - Portion approximately 2 tablespoons of brownie dough per cookie. Flatten each portion in palm of hand to create a disc. Place one chilled cookie dough ball in center and carefully wrap brownie dough around it, sealing edges completely to enclose filling. Repeat with remaining dough and filling.
07 - Arrange stuffed cookies on prepared baking sheets, spacing 2 inches apart. Bake for 10-12 minutes until tops are set with characteristic crackled surface. Centers should still appear slightly soft. Do not overbake to maintain fudgy texture.
08 - Let cookies rest on baking sheets for 10 minutes to finish setting. Carefully transfer to wire cooling rack and cool completely before serving, about 30 additional minutes.