01 - In a small mixing bowl, cream softened butter and brown sugar until light and fluffy using a hand mixer or sturdy whisk. Incorporate milk and vanilla extract until smooth. Gradually stir in heat-treated flour and salt until fully combined. Gently fold in mini chocolate chips. Portion dough into 12 equal balls, approximately 2 teaspoons each. Arrange on parchment-lined plate and refrigerate until firm, about 15-20 minutes.
02 - Set up a heatproof bowl over a saucepan of simmering water, ensuring bowl does not touch water. Add butter and chopped dark chocolate. Stir continuously until completely melted and smooth. Remove from heat and allow to cool for 5 minutes. Alternatively, microwave in 30-second intervals, stirring between each burst.
03 - Using a stand or hand mixer, beat granulated sugar, brown sugar, and eggs on medium-high speed for 2 minutes until mixture becomes pale and significantly thickened. Add vanilla extract and pour in cooled melted chocolate. Mix on low speed until fully incorporated, scraping down bowl sides as needed.
04 - In a separate medium bowl, sift together flour, cocoa powder, baking powder, and salt. Gradually add dry ingredients to wet mixture, folding with a spatula until thick, fudgy dough forms. Refrigerate for 10-15 minutes to firm slightly for easier handling.
05 - Position oven racks in center positions. Preheat to 350°F. Line two large rimmed baking sheets with parchment paper, ensuring complete coverage.
06 - Scoop approximately 2 tablespoons of brownie dough per cookie. Flatten dough between palms to create a disc. Place one chilled cookie dough ball in center. Carefully wrap brownie dough around filling, sealing edges completely. Repeat with remaining dough, ensuring no filling is visible.
07 - Arrange formed cookies on prepared sheets, spacing 2 inches apart to allow for spread. Bake for 10-12 minutes until tops appear set with characteristic crackled surface. Centers should still feel slightly soft when touched. Avoid overbaking to maintain fudgy interior texture.
08 - Remove baking sheets from oven and let cookies rest on pans for 10 minutes. During this time, cookies will finish cooking and set. Transfer to wire rack to cool completely before storing or serving.