Truly, Crazy, and Deep Chocolate: Pools of rich salty caramel separate the layers of chocolate cake, which are then coated in dark chocolate frosting and dusted with flaky salt. Prepare the wonderful cake layers and caramel up to three days before assembly (cooling actually improves texture for stacking), so you can make it effortless.
Make the cake: Preheat the oven to 350 F. Butter three 9-inch round cake pans. Line with parchment; parchment of butter. Sprinkle with cocoa, patting the excess. In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder and salt with an electric mixer on low speed until just combined. Increase the speed to medium and add the eggs, buttermilk, lukewarm water, oil and vanilla; beat until smooth, about 3 minutes.
Distribute the dough evenly among the prepared molds, smoothing the top with an angled spatula. Bake in oven until cakes are set and a toothpick comes out clean, about 35 minutes. Transfer the mussels to a wire rack to cool for 15 minutes. Unmold the cakes on a wire rack to cool completely.
Make the caramel: In a medium saucepan over high heat, combine the sugar, corn syrup and 1/4 cup of water. Cook, without stirring, until mixture is dark amber, about 14 minutes. Remove from heat and carefully pour in the cream (the mixture will splash); stir until smooth. Return to high heat and cook until a candy thermometer reaches 238 F, soft ball stage, about 2 minutes. Pour the caramel into a medium bowl, stir in the salt and let cool slightly, about 15 minutes. Stir in the butter, 1 tbsp at a time. Let cool completely.
Meanwhile, prepare the frosting: in a small heatproof bowl, whisk together the cocoa and boiling water until the cocoa is dissolved. Using an electric mixer, beat butter, icing sugar and salt on medium-high speed until pale and fluffy, 3 to 5 minutes. Reduce speed to low; add the melted and cooled chocolate and beat until well blended, scraping the sides of the bowl as needed. Add the cocoa mixture and beat until completely smooth. (The frosting can be stored in the refrigerator for up to 5 days; bring to room temperature and beat before using.)
Using a serrated knife, cut tops of cake layers to level; divide each in half horizontally to make a total of 6 layers. Transfer a layer to a serving platter or cake stand lined with strips of parchment paper and spread 3/4 cup of the caramel on top. Top with another cake layer and repeat with the remaining caramel and cake layers, leaving the top layer uncovered. Refrigerate until set, about 1 hour. Spread a thin layer of frosting over cake to form a layer of breadcrumbs; refrigerate until stiff, about 30 minutes. Spread frosting evenly over top and sides of cake in a swirling, plunging motion. Sprinkle with sea salt.
Reprinted from Martha Stewart’s Cake Perfection. Copyright © 2020 by Martha Stewart Living Omnimedia, Inc. Posted by Clarkson Potter, an imprint of Penguin Random House, LLC.