Make the Salted Caramel:

1. Melt the Sugar: In a medium saucepan over medium heat, melt the granulated sugar, stirring constantly with a heatproof spatula or wooden spoon. It will start to clump before melting into an amber-colored liquid.
2. Add Butter: Once fully melted, carefully add the butter, stirring continuously. The mixture will bubble up, so be cautious.
3. Pour in the Cream: Slowly add the heavy cream while stirring. The caramel will bubble vigorously but will smooth out as you stir.
4. Add Salt: Remove from heat and stir in the sea salt. Let the caramel cool slightly before using.

Prepare the Brownie Batter:

5. Preheat the Oven: Preheat your oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving some overhang for easy removal.
6. Melt Butter and Chocolate: In a heatproof bowl, combine the chopped dark chocolate and melted butter. Microwave in 30-second intervals, stirring between each, until smooth.
7. Whisk in Sugars: Add the granulated sugar and brown sugar to the chocolate mixture, stirring until combined.
8. Add Eggs and Vanilla: Whisk in the eggs one at a time, then stir in the vanilla extract until the mixture is glossy and thick.
9. Incorporate Dry Ingredients: Gently fold in the flour, cocoa powder, and salt, mixing just until combined. Do not overmix.

Assemble and Bake:

10. Layer the Batter: Pour half of the brownie batter into the prepared pan, spreading it evenly.
11. Add Caramel Swirl: Drizzle half of the salted caramel over the brownie layer, then top with the remaining batter. Use a knife to gently swirl the caramel through the batter.
12. Bake: Bake for 35-40 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
13. Cool: Let the brownies cool in the pan for 15 minutes before sprinkling with extra sea salt. Transfer to a wire rack to cool completely before slicing.