01 - In a small bowl, cream the softened butter and brown sugar until light and fluffy, approximately 2 minutes. Whisk in the milk and vanilla extract until smooth. Fold in the heat-treated flour and salt until fully incorporated, then gently mix in the mini chocolate chips. Portion into 12 equal balls, about 2 teaspoons each. Arrange on a parchment-lined plate and refrigerate until firm, about 20 minutes.
02 - Combine butter and chopped dark chocolate in a heatproof bowl. Microwave in 30-second intervals, stirring between each burst, until completely smooth. Alternatively, melt over a double boiler, stirring constantly. Allow the mixture to cool for 5 minutes to prevent scrambling the eggs.
03 - In a large mixing bowl, whisk together granulated sugar, brown sugar, and eggs on medium speed for 2 minutes until pale and creamy. Add vanilla extract and the cooled chocolate mixture. Beat on low speed until fully combined and smooth.
04 - In a separate medium bowl, sift together flour, cocoa powder, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, folding with a spatula just until a thick, fudgy dough forms. Do not overmix. Refrigerate the dough for 15 minutes to firm slightly for easier handling.
05 - Preheat your oven to 350°F. Line two large baking sheets with parchment paper or silicone baking mats. Set aside until ready to bake.
06 - Scoop approximately 2 tablespoons of chilled brownie cookie dough per cookie. Flatten each portion between your palms into a disc shape. Place one frozen cookie dough ball in the center and carefully wrap the brownie dough around it, sealing completely to encase the filling. Repeat with remaining dough and filling.
07 - Arrange the stuffed cookies on prepared baking sheets, spacing 3 inches apart to allow for spreading. Bake for 10 to 12 minutes, until the tops appear set with characteristic crackling across the surface. The centers should still feel slightly soft when touched lightly. Avoid overbaking to maintain the fudgy texture.
08 - Let the cookies rest on the baking sheets for 10 minutes to set. Carefully transfer to a wire cooling rack and allow to cool completely, about 30 minutes, before serving. The centers will firm as they cool.