Make the Salted Caramel:

1. Prepare your mise en place. Have butter, cream, salt, and vanilla measured and ready beside the stove. Caramel moves quickly once it starts, so preparation is key.
2. Caramelize the sugar. In a medium heavy-bottomed saucepan, heat granulated sugar over medium heat. Let it sit undisturbed for 3-4 minutes until edges begin to melt. Then stir constantly with a wooden spoon, breaking up any clumps. Continue cooking until all sugar is melted and amber-colored, about 6-8 minutes total. Watch carefully as it can burn quickly.
3. Add the butter. Remove from heat and immediately add butter pieces. The mixture will bubble vigorously—this is normal. Whisk until butter is completely melted and incorporated.
4. Add the cream. Slowly pour in heavy cream while whisking constantly. The caramel will bubble up dramatically. Continue whisking until smooth and combined.
5. Finish the caramel. Stir in sea salt and vanilla extract. Return to medium heat and cook for 1-2 minutes, stirring constantly, until smooth. Remove from heat and let cool for 10 minutes. The caramel will thicken as it cools. Reserve ½ cup for swirling; save the rest for drizzling after baking.

Make the Brownies:

6. Prepare the pan. Preheat oven to 350°F. Line a 9×13-inch baking pan with parchment paper, leaving overhang on two sides for easy removal. Lightly grease the parchment.
7. Melt chocolate and butter. In a medium saucepan over low heat, melt butter and chopped chocolate together, stirring frequently until smooth. Remove from heat and let cool slightly for 5 minutes.
8. Mix in sugar. Pour the chocolate mixture into a large mixing bowl. Whisk in sugar until well combined and slightly glossy.
9. Add eggs and vanilla. Add eggs one at a time, whisking well after each addition. The mixture should become thick and glossy. Whisk in vanilla extract.
10. Incorporate dry ingredients. In a separate bowl, sift together flour, cocoa powder, and salt. Fold the dry ingredients into the chocolate mixture using a wooden spoon or spatula. Mix just until no flour streaks remain—don’t overmix.
11. Layer and swirl. Pour brownie batter into prepared pan, spreading evenly. Drizzle the ½ cup of reserved salted caramel over the top in lines. Use a butter knife or skewer to swirl the caramel into the batter, creating a marbled pattern. Don’t over-swirl or the caramel will disappear into the batter.
12. Bake to perfection. Bake for 30-35 minutes, until edges are set and a toothpick inserted in the center comes out with a few moist crumbs (not wet batter). The center should look slightly underdone—it will continue cooking as it cools. Don’t overbake or brownies will be dry.
13. Cool completely. Remove from oven and immediately sprinkle with flaky sea salt. Let cool in the pan on a wire rack for at least 1 hour. For cleanest cuts, refrigerate for 2 hours before slicing.
14. Cut and serve. Use the parchment overhang to lift brownies from the pan. Cut into squares with a sharp knife, wiping the blade between cuts. Drizzle with remaining salted caramel before serving if desired.