For the caramel sauce:
1 cup sugar
¼ cup water
2 tbsp. light corn syrup
½ cup heavy cream, slightly warmed
1 tsp. fleur de sel
¼ cup sour cream
For the brownies:
1¼ cups all-purpose flour
1 tsp. coarse salt
2 tbsp. Dutch-process cocoa powder
11 oz. bittersweet chocolate, coarsely chopped
1 cup (2 sticks) unsalted butter, cut into 1-inch pieces
1½ cups sugar
½ cup brown sugar, firmly packed
5 large eggs, at room temperature
2 tsp. vanilla extract
Splash of Kahlua (optional)
1 tsp. fleur de sel, to finish
1 tsp. coarse sugar, to finish
To make the caramel sauce, combine the sugar, corn syrup, and water in a small saucepan set over medium-high heat. Cook, stirring frequently, until the mixture becomes deep amber in color and reaches 350? F on a candy thermometer. Remove the pan from the heat and slowly pour in the cream in a steady stream down the side of the pan, whisking constantly to incorporate before adding more. Once all the cream has been incorporated, stir in the fleur de sel and sour cream. Set aside to cool while you prepare the brownie batter.
Preheat the oven to 350? F. Line a 9 x 13″ baking pan with parchment paper or lightly greased foil. In a small bowl, combine the flour, salt and cocoa powder. Whisk to blend well. In a large microwave safe bowl, combine the chopped chocolate and butter. Microwave in short intervals (about 30-35 seconds), stirring in between, until everything is melted and the mixture is smooth. Whisk in the sugars and then the eggs. Whisk in the vanilla and Kahlua, if using. Mix in the dry ingredients, stirring just until combined and no streaks remain.
Spread about half of the brownie batter into the prepared pan and smooth with an offset spatula. Drizzle the caramel over the top in an even layer. Top with the remaining brownie batter, smoothing gently over the caramel and completely covering it. Bake for 30 minutes, rotating the pan halfway through baking. Remove to a wire rack and let cool. Combine the fleur de sel and coarse sugar in a small bowl and sprinkle about half of it over the top of the brownies. Add the remainder to taste. Let cool completely before slicing and serving. Pawn off on friends to prevent binging.