Prepare the Oven and Pan: Begin by preheating the oven to 350°F. Line a square baking pan with parchment paper so the brownies lift out easily later. Lightly grease the paper so the batter spreads smoothly and the edges bake evenly.
Mix the Dry Ingredients: Place flour, cornstarch, sea salt, and cocoa powder into a medium bowl. Whisk everything together until the mixture looks evenly combined. This step helps distribute the cocoa powder so the brownie batter stays smooth and rich.
Melt the Chocolate and Butter: Add the chopped unsweetened chocolate, semisweet chocolate, and butter into a heat safe bowl. Warm the mixture gently over a double boiler or in short microwave bursts, stirring until the chocolate melts completely. Allow it to cool slightly before moving forward.
Combine the Brownie Batter: Pour the melted chocolate mixture into a large bowl. Stir in granulated sugar and brown sugar until the mixture becomes glossy. Add the eggs one at a time while mixing gently, then stir in vanilla extract. Finally fold the dry ingredients into the batter until everything comes together into a thick chocolate mixture.
Bake the Brownies: Spread the brownie batter evenly into the prepared pan. Smooth the surface with a spatula so the layer bakes evenly. Place the pan in the oven and bake for about 25 to 30 minutes until the center looks set and a toothpick inserted shows moist crumbs. Allow the brownies to cool completely before adding the topping.
Prepare the Cookie Dough Frosting: Place softened butter and brown sugar in a bowl and mix until creamy and smooth. Add vanilla extract and milk, then stir until the texture becomes light and spreadable. Next add the heat treated flour and sea salt while mixing gently until the dough forms.
Add Chocolate Chips: Fold mini chocolate chips into the cookie dough mixture. Stir just enough so the chips spread evenly throughout the dough. The dough should feel soft and slightly thick but easy to spread across the brownies.
Assemble and Slice: Spread the cookie dough frosting evenly over the completely cooled brownie layer. Smooth the surface gently with a spatula so the topping forms a thick, even layer. Chill the pan for about 30 minutes before slicing into squares for clean edges.
