Truly, madly and deeply chocolate: Pools of rich salted caramel separate layers of chocolate cake, which are then swathed with dark chocolate frosting and sprinkled with flaky salt. Prep the wondrous cake layers and caramel up to three days before assembling (chilling actually improves the texture for stacking), so you can make it look effortless.
Make the cake: Preheat oven to 350 F. Butter three 9-inch round cake pans. Line with parchment; butter parchment. Dust with cocoa, tapping out any excess. In a large bowl, whisk together flour, sugar, cocoa, baking soda, baking powder and salt with an electric mixer on low speed until just combined. Increase speed to medium and add eggs, buttermilk, warm water, oil and vanilla; beat until smooth, about 3 minutes.2.
Divide batter evenly among prepared pans, smoothing tops with an offset spatula. Bake until cakes are set and a cake tester comes out clean, about 35 minutes. Transfer pans to a wire rack to cool 15 minutes. Turn out cakes onto rack to cool completely.3.
Make the caramel: In a medium saucepan over high heat, combine sugar, corn syrup and 1/4 cup water. 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 high heat and cook until a candy thermometer reaches 238 F, soft-ball stage, about 2 minutes. Pour caramel into a medium bowl, stir in salt and let cool slightly, about 15 minutes. Stir in butter, 1 tablespoon at a time. Let cool completely.4.
Meanwhile, make the frosting: In a small heatproof bowl, whisk together cocoa and boiling water until cocoa is dissolved. With an electric mixer, beat butter, confectioners’ sugar and salt on medium-high speed until pale and fluffy, 3 to 5 minutes. Reduce speed to low; add melted and cooled chocolate and beat until combined, scraping down sides of bowl as needed. Add cocoa mixture and beat until completely smooth. (Frosting can be stored in refrigerator up to 5 days; bring to room temperature and beat before using.)5.
With a serrated knife, trim tops of cake layers to level; split each in half horizontally to form a total of 6 layers. Transfer a layer to a serving plate or cake stand lined with parchment strips and spread 3/4 cup caramel over top. Top with another cake layer and repeat with remaining caramel and cake layers, leaving top layer uncovered. Refrigerate until set, about 1 hour. Spread a thin layer of frosting over cake to form a crumb coat; refrigerate until firm, about 30 minutes. Spread frosting evenly over top and sides of cake in a swirling, swooping motion. Sprinkle with sea salt.
Reprinted from Martha Stewart's Cake Perfection. Copyright © 2020 by Martha Stewart Living Omnimedia, Inc. Published by Clarkson Potter, an imprint of Penguin Random House, LLC.