01 - Heat oven to 350°F. Line an 8x8-inch baking pan with parchment paper leaving overhangs for easy removal.
02 - In a medium saucepan over medium heat, melt granulated sugar, stirring continuously until amber in color. Incorporate cubed butter, whisking until melted. Remove from heat and carefully whisk in heavy cream; it will bubble. Stir in flaky sea salt and let cool to room temperature.
03 - Combine melted butter, granulated sugar, and brown sugar in a large bowl, whisking until smooth. Beat in eggs and vanilla extract until mixture is glossy.
04 - Sift together flour, cocoa powder, and sea salt; fold into the wet mixture until just combined. Stir in chopped dark chocolate.
05 - Transfer batter to the prepared pan and smooth surface. Drizzle cooled salted caramel over the batter and gently swirl using a knife or skewer to create a marbled effect.
06 - Bake for 28 to 32 minutes, testing doneness with a toothpick which should come out with a few moist crumbs.
07 - Allow brownies to cool completely in the pan before removing and cutting into 12 squares.