For the Salted Caramel Filling:
1 cup granulated sugar
2 tablespoons light corn syrup
½ cup heavy cream
1 teaspoon fleur de sel
¼ cup sour cream
For the Brownie Batter:
1¼ cups all-purpose flour
1 teaspoon table salt
2 tablespoons dark unsweetened cocoa powder
11 ounces dark chocolate, coarsely chopped
1 cup unsalted butter, cut into 1-inch cubes
1½ cups granulated sugar
½ cup light brown sugar
5 eggs, at room temperature
2 teaspoons vanilla extract
For the Topping:
1½ teaspoons fleur de sel
1 teaspoon coarse sugar
1. Make the Caramel: In a medium saucepan, combine the sugar and corn syrup with ¼ cup water, stirring them together carefully so you don't splash the sides of the pan. Cook over high heat until an instant-read thermometer reads 350 degrees F, or until the mixture is dark amber in color (keep a close eye on the caramel at all times, as it goes from golden brown to black and burnt very quickly), 6 to 8 minutes. Remove from the heat, and slowly add the cream (careful, it will bubble up) and then the fleur de sel. Whisk in the sour cream. Set aside to cool.
2. Make the Brownie Batter: Preheat oven to 350 degrees F.
3. Butter the sides and bottom of a glass or light-colored metal 9x13-inch pan. Line the bottom with a sheet of parchment paper, and butter the parchment.
4. In a medium bowl, whisk together the flour, salt and cocoa powder.
5. Place the chocolate and butter in the bowl of a double boiler set over a pan of simmering water, and stir occasionally until the chocolate and butter are completely melted and combined. Turn off the heat, but keep the bowl over the water of the double boiler, and add both sugars. Whisk until completely combined and remove the bowl from the pan. The mixture should be at room temperature.
6. Add three eggs to the chocolate mixture and whisk until just combined. Add the remaining eggs and whisk until just combined. Add the vanilla and stir until combined. Do not overbeat the batter at this stage, or your brownies will be cakey.
7. Sprinkle the flour mixture over the chocolate. Using a spatula, fold the dry ingredients into the wet ingredients until there is just a trace amount of the flour mixture still visible.
8. Assemble the brownies: Pour half of the brownie mixture into the pan and smooth the top with a spatula. Drizzle about ¾ cup of the caramel sauce over the brownie layer in a zigzag pattern, making sure the caramel doesn't come in contact with the edges oft he pan or it will burn. Use an offset spatula to spread the caramel evenly across the brownie layer, leaving about a ½-inch border around the edges. In heaping spoonfuls, scoop the rest of the brownie batter over the caramel layer. Smooth the brownie batter gently to cover the caramel layer.
9. Bake the brownies for 30 minutes, rotating the pan halfway through the baking time, and check to make sure the brownies are completely done by sticking a toothpick into the center of the pan. The brownies are done when the toothpick comes out with a few moist crumbs.
10. Remove the brownies from the oven and sprinkle with the fleur de sel and coarse sugar.
11. Cool the brownies completely before cutting and serving.
12. The brownies can be stored, tight wrapped at room temperature, for up to 4 days.