Instructions
1_ Preheat oven to 350 degrees. Make the cake: Butter three 9-inch round cake pans, and dust with flour, tapping out excess. Sift flour, granulated sugar, cocoa, baking soda, baking powder, and 1 1/2 teaspoons coarse salt into the bowl of a mixer. Beat on low speed until just combined. Raise speed to medium, and add eggs, buttermilk, 1 1/2 cups warm water, oil, and vanilla. Beat until smooth, about 3 minutes.
2_ Divide batter among pans. Bake until cakes are set and a toothpick inserted into the center of each comes out clean, about 35 minutes. Let cool in pans set on a wire rack for 15 minutes. Turn out cakes onto racks, and let cool completely.
3_ Make the caramel: Combine granulated sugar, corn syrup, and 1/4 cup water in a medium saucepan over high heat. Cook, without stirring, until mixture is dark amber, about 14 minutes. Remove from heat, and carefully pour in cream (mixture will spatter); stir until smooth. Return to heat, and cook until a candy thermometer reaches 238 degrees, about 2 minutes. Pour caramel into a medium bowl, stir in 1 teaspoon coarse salt, and let cool slightly, about 15 minutes. Stir in butter, 1 tablespoon at a time. Let cool completely.
4_ Meanwhile, make the frosting: Whisk together cocoa and 1/4 cup plus 2 tablespoons warm water in a bowl until cocoa dissolves. Beat butter, confectioners’ sugar, and a generous pinch of coarse salt in a clean bowl with a mixer on medium speed until pale and fluffy. Gradually beat in melted chocolate and then cocoa mixture until combined. Let stand for 30 minutes before using.
5_ Trim tops of cakes using a serrated knife to create a level surface. Cut each in half horizontally to form 2 layers. Transfer 1 layer to a serving platter, and spread 3/4 cup caramel over top. Top with another cake layer, and repeat with remaining caramel and cake layers, leaving top uncovered. Refrigerate until set, about 1 hour.
6_ Frost top and sides of cake in a swirling motion. Sprinkle with sea salt.