Preheat your oven to 350°F (175°C). Line an 8x8-inch metal baking pan with parchment paper, leaving an overhang on the sides.
Melt the butter and chopped dark chocolate in a heatproof bowl over simmering water until completely smooth. Remove from heat and let cool slightly.
In a separate large bowl, whisk the eggs, granulated sugar, and dark brown sugar vigorously for 2-3 minutes until pale and thick.
Gently fold the melted chocolate mixture into the whipped eggs and sugar. Stir in the vanilla extract.
Sift the flour, cocoa powder, and table salt over the wet ingredients. Fold gently with a spatula just until no dry flour remains.
In a microwave-safe bowl, melt the soft caramels with the heavy cream in 15-second intervals, stirring until smooth and pourable.
Pour half of the brownie batter into the prepared pan. Pour 80% of the melted caramel evenly over the batter. Dollop the remaining batter on top, then drizzle with the rest of the caramel. Use a knife to gently swirl the layers together.
Bake for 30-35 minutes until the edges are set and a toothpick inserted in the center comes out with moist, thick crumbs.
Remove from the oven, immediately sprinkle with flaky sea salt, and let cool completely in the pan for at least 2 hours before slicing.
