Preheat the oven to 350°F (175°C). Lightly coat a 9×13-inch metal baking pan with nonstick spray. Line it with parchment paper, allowing some overhang for easy lifting later.

Melt the butter in a large mixing bowl. Stir in the brown and granulated sugars until well combined. Whisk in the eggs and vanilla until smooth, about 30 to 45 seconds. Gradually mix in the dry ingredients until the dough forms, then fold in the chocolate chips. Divide the dough evenly into two portions (roughly 640 grams each). Press one portion firmly into the bottom of the prepared pan, then carefully lift it out with the parchment paper and set it aside. Line the pan with a fresh piece of parchment and leave the remaining dough in the bowl for later.

In a medium saucepan, melt the cubed butter over medium heat. Once fully melted, reduce the heat to medium-low and stir frequently. The butter will foam and sizzle before taking on a deep golden hue with a nutty, caramel-like aroma. This process takes about 3 to 6 minutes. Remove from heat and immediately stir in the chopped semi-sweet chocolate until fully melted.

In a separate large mixing bowl, whisk together the eggs, egg yolk, and granulated sugar for 1 to 2 minutes, until the sugar dissolves completely. To test, rub a small amount between your fingers—if you still feel sugar granules, keep whisking. Once smooth, mix in the powdered sugar, vanilla extract, and salt. Let the chocolate and butter mixture cool slightly before incorporating it into the eggs and sugar mixture. Stir in the cocoa powder and vegetable oil until combined. Finally, fold in the flour, espresso powder (if using), and chocolate chips.

Pour just over half of the brownie batter into the parchment-lined pan, spreading it evenly. Gently lay the reserved sheet of cookie dough over the brownie layer, then carefully remove the parchment paper. Spread the remaining brownie batter over the top. Shape the remaining cookie dough into large discs and arrange them over the surface. If desired, press extra chocolate chips into any exposed cookie dough.

Transfer the pan to the oven and bake for 38 to 44 minutes. The top should appear glossy, with the cookie dough taking on a golden hue. To check for doneness, insert a toothpick into a brownie section—moist crumbs should cling to it, but there should be no wet batter. Avoid overbaking to maintain the ideal chewy texture.

Let the brookies cool in the pan on a wire rack for 1 to 2 hours before slicing. If desired, sprinkle with flaky sea salt for an added touch of flavor.

Once completely cooled, store the brookie bars in an airtight container at room temperature for up to 3 to 4 days. For the best experience, warm them slightly in the microwave before serving.