1-Prepare the brown butter: Gently heat the butter until it turns a rich amber and brown specks are visible. Reserve about 90 grams of the brown butter, adding a tablespoon of water if it solidifies to ease mixing.

2-Make the brownie batter: Preheat the oven to 350°F (175°C). Butter and line an 8-inch square metal baking pan with parchment for easy removal. Melt the remaining butter with 180 grams of semi-sweet chocolate chips until smooth. Stir in cocoa powder until combined.

3-Mix eggs and sugar: In a separate bowl, whisk together the eggs and granulated sugar vigorously until light, fluffy, and aerated; this encourages a crinkle top.

4-Combine brownie ingredients: Fold the egg mixture into the melted chocolate mixture. Gently fold in the flour and salt just until mixed, being careful not to overwork the batter.

5-Prepare the cookie dough: Whisk the reserved brown butter with dark brown and granulated sugars until smooth. Add the egg and vanilla extract, stirring well. Fold in flour, baking powder, baking soda, and salt until no streaks remain. Finally, fold in cookie dough chocolate chips.

6-Assemble the bars: Spread half of the brownie batter evenly in the prepared pan. Press two-thirds of the cookie dough over the brownie layer in an even layer. Pour the remaining brownie batter on top and smooth. Scatter remaining cookie dough pieces over the surface.

7-Add toppings: Optionally, sprinkle extra semi-sweet chocolate chips and flaky salt over the top for extra flavor and texture contrast.

8-Bake: Bake 40 to 45 minutes until edges are set and the center is slightly wobbly, indicating a fudgy texture. Tent loosely with foil after 25 minutes if the cookie topping browns too quickly, removing foil for last 5-10 minutes.

9-Cool and serve: Allow bars to cool completely before slicing to prevent crumbling and to set the layers perfectly.