Melt 1 cup (2 sticks) unsalted butter in a light-colored saucepan over medium heat. Swirl occasionally as it foams and sizzles. After 5 to 6 minutes, amber-golden browned milk solids will settle at the bottom and release an intoxicating hazelnut aroma. Pour into a heatproof bowl to cool for 10 minutes. Divide equally: 1/2 cup for the cookies, 1/2 cup for the brownies.

Preheat your oven to 350°F (175°C). Line an 8x8-inch metal baking pan with parchment paper, leaving an overhang on the sides for easy lifting.

In a medium bowl, whisk 1/2 cup browned butter, brown sugar, and granulated sugar for 1 minute until smooth. Whisk in the egg and vanilla extract until glossy.

Fold in the flour, baking soda, and salt with a spatula until just combined. Gently fold in chocolate chips. Press the dough evenly across the bottom of the prepared pan.

In a microwave-safe bowl, combine the remaining 1/2 cup warm browned butter and chopped bittersweet chocolate. Melt in 20-second bursts, stirring until silky and glossy.

In a separate bowl, vigorously whisk the 2 eggs and sugar with a hand mixer or balloon whisk for full 3 minutes until pale, thick, and ribbony (this creates the signature crackly crinkle top!). Whisk in the melted chocolate mixture and vanilla.

Gently fold in sifted cocoa powder, flour, and salt until a luxurious, thick brownie batter forms.

In a small dish, stir 1 tablespoon flour into the thick salted caramel sauce. Drizzle half of the caramel over the raw cookie dough base. Spread the rich brownie batter evenly over the top, then swirl the remaining caramel into the surface.

Bake at 350°F (175°C) for 32 to 36 minutes, until the top is shiny and crackled and the edges are set. The center should still have a slight decadent wobble.

Immediately sprinkle generously with flaky Maldon sea salt. Allow to cool completely in the pan at room temperature for 1 hour, then chill in the refrigerator for 1 hour before slicing into 16 thick, fudgy, decadent squares!