Preheat oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.
In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth and glossy, about 1 minute.
Add eggs one at a time, whisking vigorously after each addition until the mixture is thick and pale. Stir in vanilla extract.
Sift cocoa powder directly into the bowl, then add flour, fine sea salt, and baking powder. Fold gently with a spatula until just combined and no dry streaks remain — do not overmix.
Pour batter into prepared pan and spread evenly with an offset spatula. Dollop warmed caramel sauce in 5-6 lines across the surface.
Using a butter knife or skewer, swirl the caramel through the batter in a figure-8 motion 3-4 times — stop before it becomes fully mixed. Sprinkle flaky sea salt over the top.
Bake for 28-32 minutes, until edges are set and a toothpick inserted in the center comes out with moist crumbs (not wet batter). The center should look slightly underdone.
Cool completely in the pan on a wire rack for at least 2 hours before lifting out and cutting into 24 squares. For cleanest cuts, chill for 30 minutes and wipe your knife between slices.
