Preheat oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.

Make the salted caramel: In a medium saucepan over medium heat, melt 1/2 cup sugar without stirring until it turns deep amber, about 6-8 minutes. Swirl the pan occasionally to ensure even melting.

Remove from heat and immediately whisk in the cubed butter until smooth. Slowly pour in the heavy cream while whisking (it will bubble vigorously). Stir in 1/2 teaspoon sea salt. Let cool to room temperature, about 30 minutes.

In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth and glossy. Add eggs one at a time, whisking well after each addition, then stir in vanilla extract.

Sift the cocoa powder directly into the wet ingredients, then add flour and salt. Fold gently with a spatula just until no dry streaks remain. Do not overmix.

Pour brownie batter into the prepared pan and spread evenly. Drizzle the cooled caramel over the top in parallel lines.

Use a butter knife or wooden skewer to swirl the caramel through the batter in figure-8 motions, creating a marbled pattern. Sprinkle with flaky sea salt.

Bake for 28-32 minutes, until edges are set and the center still jiggles slightly when gently shaken. A toothpick inserted in the center should have moist crumbs, not wet batter.

Cool completely in the pan on a wire rack for at least 2 hours. Use the parchment overhang to lift out, then cut into 12-16 squares with a sharp knife, wiping the blade between cuts.