Preheat oven to 350°F (175°C). Line a 9x9-inch metal baking pan with parchment paper, leaving an overhang on the sides for easy lifting. Lightly butter the parchment.

In a heatproof bowl set over a saucepan of simmering water (or in 30-second microwave bursts), melt the chopped dark chocolate and cubed butter together, stirring until glossy and smooth. Set aside to cool slightly.

In a large mixing bowl, beat room temperature eggs, granulated sugar, and brown sugar with a hand mixer or stand mixer on medium-high speed for a full 4 to 5 minutes until pale, thick, and ribbon-like. This step dissolves the sugar and creates the shiny papery meringue crust!

Pour the melted chocolate-butter mixture and vanilla extract into the whipped eggs, gently folding with a rubber spatula until just combined.

Sift in the flour, Dutch-process cocoa powder, and fine sea salt. Fold gently until no dry streaks remain—do not overmix.

Spread two-thirds of the thick brownie batter into the prepared pan. Whisk 1 tablespoon of flour into the salted caramel sauce, then drizzle half over the batter. Spread remaining brownie batter on top and dollop the rest of the caramel. Use a butter knife or skewer to create elegant figure-eight swirls across the surface.

Bake at 350°F for 30 to 35 minutes until the edges are set and a toothpick inserted into the center comes out with a few moist, fudgy crumbs (not wet batter).

Immediately sprinkle Maldon flaky sea salt over the warm top. Cool completely in the pan for at least 2 hours (or chill 1 hour in the fridge) before slicing into 16 ultra-fudgy squares with clean knife cuts!