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1
To prepare cake: Preheat oven to 325u00b0. Butter three 8-inch round cake pans. Line the bottom of each with parchment paper, and butter the parchment. Dust with flour and knock out the excess.
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2
Combine cocoa powder, hot water, and sour cream. Set aside to cool. Sift flour, baking power, baking soda, and salt together. Set aside.
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3
Using an electric mixer, beat butter and shortening on medium speed until ribbon-like, about 5 minutes. Add both sugars and beat until light and fluffy, about 5 minutes. Add eggs, one at a time, and beat well after each addition. Add vanilla and mix well. Scrape down sides of bowl and mix again for 30 seconds. Add flour mixture, alternating with cocoa mixture, in three portions, beginning and ending with the flour mixture.
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4
Divide batter evenly among prepared pans and smooth the tops. Bake 35-40 minutes, rotating pans halfway through. Cakes are done when a toothpick inserted in the center of each cake comes out clean.
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5
Cool cakes in pans for 20 minutes on wire racks. Remove from pans and let cool completely. Remove parchment paper.
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6
To Prepare the Salted Caramel: In a small saucepan, combine cream and fleur de sel. Bring to a simmer over very low heat until salt has dissolved. Be careful not to let the cream burn.
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7
In a medium saucepan, combine 1/4 cup water, sugar, and corn syrup, stirring carefully so as not to splash the sides of the pan. Cook over high heat about 6-8 minutes, until an instant-read thermometer reads 350u00b0. Remove from heat and let cool for 1 minute.
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8
Add cream mixture to sugar mixture. Whisk in sour cream. Let caramel cool to room temperature, then transfer to an airtight container and refrigerate until time to assemble the cake.
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9
To Prepare the Caramel Ganache: Put the chocolate in a large heatproof bowl and set aside.
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10
Bring the cream to a simmer over very low heat. Make sure cream does not burn.
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11
In a medium saucepan, combine 1/4 cup water, sugar, and corn syrup, stirring carefully so as not to splash the sides of the pan. Cook over high heat for 6-8 minutes until an instant-read thermometer reads 350u00b0. Remove from heat and let caramel cool for 1 minute.
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12
Add cream to the caramel. Stir slowly for 2 minutes. Then, pour caramel over chocolate. Let sit for 1 minute. Then, starting in the center of the bowl, and working out to the edges, slowly stir in a circle until chocolate is completely melted. Let cool, then transfer to the bowl of an electric mixer. Mix on low speed until the bowl feels cool to the touch. Increase speed to medium-high and gradually add butter, beating until thoroughly incorporated. Scrape down sides and beat on high speed until mixture is fluffy.
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13
To Assemble the Cake: Place one cake layer on serving platter. Spread 1/4 cup caramel over the top, letting it soak into the cake. Spread 3/4 cup of ganache frosting over the caramel. Sprinkle 1 teaspoon fleur de sel over the frosting. Top with the second cake layer. Spread with frosting and sprinkle with 1 teaspoon fleur de sel. Top with the third cake layer. Spread with caramel. Crumb coat the cake and put cake in refrigerator for 15 minutes. Frost the sides and top with the remaining frosting. Garnish with a sprinkle of fleur de sel.
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14
Cake will keep at room temperature (cool and humidity-free) for up to 3 days. May be refrigerated for up to 3 days. Allow cake to sit at room temperature for at least 2 hours before serving.
