How to Make Caramel Filling:
Add the sugar, corn syrup, and water to a sauce pan and carefully stir it. Cook over high heat until it reaches 350 degrees or it’s a dark amber color. This will take about 8 minutes.
Remove the pan from the heat and slowly pour in the cream. It will bubble up. Add the salt and then whisk in the sour cream. Set the caramel filling aside to cool.
How to Make Salted Caramel Brownies:
Place the dark chocolate and butter in the bowl of a double boiler and set over a pan of simmering water. Stir occasionally until the chocolate and butter are fully melted and combined. Whisk in the sugar off the heat and then begin whisking in the eggs.
Add the flour to the chocolate. Use a spatula to fold the dry ingredients in to the wet ingredients until there is only a trace amount of the flour mixture visible.
Pour half of the brownie batter into a pan, top with about 3/4 of the caramel sauce, and then pour the drop spoonfuls of the remaining brownie batter over the top and gently smooth it out.
Bake the brownies for 30 minutes. Use a toothpick inserted in the center to check for doneness. The brownies are done when the toothpick comes out with a few moist crumbs. Sprinkle with fleur de sel.
