For the caramel filling, combine the sugar and corn syrup with 1/4 cup of water in a medium saucepan, stirring carefully so as not to splash the sides of the pan. Cook over high heat until an instant-read thermometer reads 350F or until the mixture is dark amber in color, 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 and set aside to cool.
For the brownie, preheat the oven to 350 degrees Fahrenheit.
Butter the sides and bottom of a glass or light coloured metal 9 x 13 inch pan. Line the bottom with a sheet of parchment paper, and butter the parchment.
In a medium bowl, whisk together the flour, salt and cocoa powder.
Place the chocolate and butter in the bowl of the 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 at this point.
Add three eggs to the chocolate mixture and whisk until just combined. Add the remaining two 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.
Sprinkle the flour mixture over the chocolate. Using a spatula, fold the dry ingredients into the wet ingredients until there is a just a trace amount of the flour mixture visible.
To assemble the brownie, pour half of the brownie mixture into the pan and smooth the top with a spatula. Drizzle about 3/4 cup of the caramel sauce over the brownie layer in a zigzag pattern, taking care to make sure the caramel does not come in contact with the edges of the pan or it will burn. (There will be caramel sauce left over that you can either serve over the brownies when serving or refrigerate and use for something else.) Use your offset spatula to spread the caramel evenly across the brownie layer. In heaping spoonfuls, scoop the rest of the brownie batter over the caramel layer. Smooth the brownie batter gently to cover the caramel layer.
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.
Remove the brownies from the oven and sprinkle with an extra 1 1/2 teaspoons Fleur de Sel and 1 teaspoon coarse sugar.
Cool the brownies completely before cutting and serving. Store wrapped tightly in plastic wrap for up to 4 days.