01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a medium saucepan over medium heat, melt sugar while stirring continuously until amber in color. Add butter and whisk until melted. Remove from heat and carefully whisk in heavy cream; mixture will bubble. Stir in flaky sea salt and allow to cool to room temperature.
03 - In a large bowl, whisk together melted butter, granulated sugar, and brown sugar until smooth. Add eggs and vanilla extract, stirring until glossy.
04 - Sift flour, cocoa powder, and sea salt into the wet mixture. Fold gently until just combined. Stir in chopped dark chocolate.
05 - Pour batter into the prepared pan and smooth the surface. Drizzle salted caramel evenly over the top, then swirl gently with a knife or skewer to create a marbled effect.
06 - Bake for 28 to 32 minutes or until a toothpick inserted in the center emerges with a few moist crumbs.
07 - Allow to cool completely in the pan before removing and cutting into 12 squares.