Step 1: Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the brownies out later. Lightly grease the parchment.
Step 2: In a large microwave-safe bowl, combine the unsalted butter and unsweetened chocolate. Microwave in 30-second intervals, stirring after each, until melted and smooth. Alternatively, melt over a double boiler.
Step 3: To the melted chocolate mixture, whisk in the granulated sugar and brown sugar until well combined. Add the eggs one at a time, whisking vigorously after each addition until glossy and smooth. Stir in the vanilla extract.
Step 4: In a separate medium bowl, whisk together the all-purpose flour, cocoa powder, and salt.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix; overmixing can lead to tough brownies. Stir in any optional chocolate chips or nuts for the brownie layer.
Step 6: Pour the brownie batter into the prepared pan and spread evenly. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs, but not wet batter. The edges should be set. Let the brownies cool completely in the pan on a wire rack.
Step 7: While the brownies cool, heat-treat your flour for the cookie dough. Spread flour on a baking sheet and bake at 300°F (150°C) for 5-7 minutes, or microwave on high for 60-90 seconds, stirring every 30 seconds, until it reaches 160°F (71°C). Let cool completely.
Step 8: In a large bowl, using an electric mixer, cream together the softened unsalted butter, granulated sugar, and brown sugar until light and fluffy. This can take 2-3 minutes.
Step 9: Beat in the milk and vanilla extract until well combined and smooth.
Step 10: Gradually add the heat-treated flour and salt to the wet mixture, mixing on low speed until just combined. Again, avoid overmixing.
Step 11: Gently fold in the semi-sweet chocolate chips until evenly distributed throughout the cookie dough.
Step 12: Once the brownies are completely cool, evenly spread the edible cookie dough mixture over the cooled brownie layer. You can use an offset spatula or your hands (lightly floured if sticky) to press it into an even layer.
Step 13: Cover the pan loosely with plastic wrap and refrigerate for at least 1-2 hours, or until the cookie dough layer is firm enough to cut cleanly.
Step 14: Once chilled, use the parchment paper overhang to lift the entire slab out of the pan. Place on a cutting board and slice into squares. Use a sharp knife and wipe it clean between cuts for the neatest edges. Enjoy!