Step 1: Preheat your oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.
Step 2: In a medium bowl, whisk together the melted butter, granulated sugar, and 1/4 cup brown sugar until well combined.
Step 3: Add the eggs and 1 tsp vanilla extract, whisking vigorously for about 1 minute until the mixture is slightly pale.
Step 4: Gently fold in the 1/2 cup flour, cocoa powder, and 1/4 tsp salt until just combined. Do not overmix.
Step 5: Pour the brownie batter into the prepared pan and bake for 22-25 minutes. A toothpick should come out with a few moist crumbs. Let the brownies cool completely in the pan.
Step 6: While the brownies cool, prepare the dough layer. In a large bowl, cream together the softened butter and 3/4 cup brown sugar until light and fluffy.
Step 7: Stir in the vanilla, heat-treated flour, salt, and milk until a smooth dough forms.
Step 8: Fold in the mini chocolate chips.
Step 9: Once the brownies are cold, spread the cookie dough evenly over the top using an offset spatula.
Step 10: Refrigerate for at least 2 hours to allow the layers to set before slicing into 16 squares.