Preheat your oven to 350°F (175°C). Line an 8x8-inch or 9x9-inch metal square baking pan with parchment paper, leaving generous overhang flaps on opposite sides for effortless lifting.

Melt the chocolate, butter, and coffee: In a heatproof bowl set over a saucepan of gently simmering water (or in short 25-second microwave bursts), melt the chopped dark chocolate, cubed butter, and instant espresso powder together. Whisk until silky smooth and glossy. Whisk in the Dutch-process cocoa powder until fully dissolved. Set aside to cool slightly.

Beat the eggs and sugar for the signature shiny crinkly top: In a large mixing bowl, beat the 3 eggs, 1 egg yolk, granulated sugar, dark brown sugar, and vanilla extract using an electric hand mixer on high speed for a full 5 to 6 minutes. The mixture must transform into a pale, thick, velvety ribbon that falls in heavy ribbons from the beaters. This step is the sole scientific secret to creating that coveted crinkly meringue crust!

Combine wet mixtures: Slowly pour the lukewarm melted chocolate-butter mixture into the whipped egg foam. Fold gently with a wide silicone spatula in wide sweeping motions just until incorporated, being careful not to deflate the trapped air.

Fold in flour: Sift the all-purpose flour and fine sea salt over the batter. Fold with the spatula just until the last streak of flour disappears. Do not overmix—stop as soon as the batter becomes thick, dark, and glossy.

Pan & swirl caramel: Pour three-quarters of the batter into the prepared pan. Spoon dollops of the thick salted caramel across the top, then cover with the remaining brownie batter. Use the tip of a small offset spatula or butter knife to gently swirl the caramel through the top layer in elegant figure-eight patterns.

Bake at 350°F for 26 to 28 minutes. A wooden toothpick inserted into the center should come out with moist, fudgy crumbs attached (not raw liquid batter, but definitely not dry and clean).

Immediately sprinkle the hot shiny top generously with flaky Maldon sea salt. Allow the brownies to cool completely in the pan at room temperature for 1.5 hours, then transfer to the refrigerator for 45 minutes to set the molten center. Lift out using the parchment wings and slice into 16 razor-sharp fudgy squares with a clean, warm knife.