Prepare the Brownie Batter
Preheat oven to 180°C (350°F).
Melt the butter and dark chocolate together in a heatproof bowl over simmering water. Stir until smooth.
In a separate bowl, whisk together sugar and eggs until smooth and glossy.
Fold in the flour and cocoa powder until just combined. Do not overmix.
Make the Salted Caramel Sauce
In a saucepan over medium heat, melt the sugar, stirring constantly.
Add butter and mix until fully combined.
Slowly pour in the heavy cream while stirring. Simmer for 2-3 minutes until thickened.
Remove from heat and stir in sea salt. Let cool slightly.
Assemble the Brownies
Pour half of the brownie batter into a greased 9x9-inch baking dish.
Drizzle half of the caramel sauce over the batter.
Pour the remaining brownie batter on top and smooth it out.
Drizzle the remaining caramel and swirl with a knife for a marbled effect.
Bake
Bake for 30-35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
Serving & Storage
Slice into 12 squares. Serve with ice cream or a drizzle of extra caramel sauce.