Preheat your oven to 350 degrees Fahrenheit and line your baking pan with parchment paper, leaving overhang for easy removal.
In a large bowl, whisk together melted butter, granulated sugar, and brown sugar until smooth and glossy.
Add eggs one at a time, whisking well after each addition. Stir in vanilla extract.
Sift in flour, cocoa powder, and salt. Gently fold until just combined. Do not overmix.
Pour half of the brownie batter into the prepared pan and spread evenly.
Warm the caramel sauce slightly so it’s pourable, then drizzle it evenly over the batter. Sprinkle lightly with sea salt.
Spoon the remaining brownie batter over the caramel layer and gently spread to cover.
Bake for 30 to 35 minutes, or until the center is set and a toothpick comes out with moist crumbs.
Let brownies cool completely before slicing. Finish with extra flaky sea salt just before serving.