1. Preheat oven to 325 degrees. Butter three 8-by-2-inch round cake pans. Line each pan with a parchment paper round, butter parchment paper and flour; set aside.

2. In a large bowl, whisk together cocoa, 1 1/4 cups hot water, and sour cream; set aside to cool, about 10 minutes.

3. In another large bowl, sift together flour, baking powder, baking soda, and salt; set aside.

4. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and shortening together until smooth and it appears to create strings inside the bowl, about 7 minutes. Add both sugars and continue beating until light and fluffy, about 7 minutes. Add eggs, one at a time, and beat until well incorporated. Add vanilla, scrape down the sides of the bowl with a spatula, and mix again for 30 seconds. Add flour mixture alternating with cocoa mixture, beginning and ending with flour mixture.

5. Divide batter evenly among the three prepared pans. Bake until cake is just firm to the touch and a toothpick inserted into the center of the cake comes out clean, 35 to 40 minutes. Let cool completely.

6. Using a serrated knife, trim tops of cakes to make level. Place four strips of parchment paper around perimeter of a serving plate or lazy Susan. Place the first layer on the cake plate. Using about 1/4 cup of the caramel, spread a thin layer on the cake, allowing some of the caramel to soak into the cake. Follow the caramel layer with a layer of about 1 cup of the ganache icing. Place the second layer on top and repeat process with another layer of caramel followed by a layer of ganache icing. Place the remaining layer on top of the second layer bottom side up. Spread entire cake with remaining ganache icing. Sprinkle with fleur de sel.