In a medium saucepan, heat the granulated sugar over medium heat. Stir occasionally with a heatproof spatula until the sugar melts into a deep amber liquid, about 8-10 minutes. Watch closely - it goes from perfect to burnt fast.

Remove from heat and immediately whisk in the butter until melted and smooth. Carefully whisk in the heavy cream (it will bubble vigorously) until fully incorporated. Stir in the sea salt. Transfer to a heatproof bowl and let cool while you make the brownies.

Preheat oven to 350 degrees F. Line an 8x8-inch pan with parchment, leaving overhang for easy removal. Combine butter and chocolate in a heatproof bowl set over simmering water. Stir until melted and smooth, then remove from heat.

Whisk both sugars into the warm chocolate mixture until combined. Add eggs one at a time, whisking well after each. Stir in vanilla. Sift flour, cocoa powder, and salt over the mixture. Fold gently with a spatula until just combined - do not overmix.

Spread half the brownie batter in the prepared pan. Drizzle half the caramel over the batter. Drop spoonfuls of remaining brownie batter on top, then drizzle remaining caramel. Use a knife to swirl gently. Sprinkle with flaky sea salt.

Bake 30-35 minutes until the top is set and a toothpick inserted in the center comes out with moist crumbs but no wet batter. Cool completely in the pan on a wire rack, at least 1 hour. Lift out using parchment overhang and cut into squares.