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Preheat oven to 350 degrees. Butter two 10-by-3-inch round cake pans; set aside.
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In a medium bowl, mix together baking soda and boiling water; add melted unsweetened chocolate and stir to combine. Set aside to cool.
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In the bowl of an electric mixer fitted with the paddle attachment, cream together butter and sugar. Add eggs, one at a time, mixing well after each addition. With the mixer on low, add flour and 6 tablespoons cocoa powder; mix until combined. Slowly add buttermilk; mix until combined. Add chocolate mixture and vanilla; mix until well combined.
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Divide mixture evenly between prepared cake pans. Transfer to oven and bake until a cake tester inserted into the center of the cake comes out clean, 45 to 50 minutes. Transfer cake pans to racks and let cool completely in pans.
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Meanwhile, heat heavy cream and semisweet chocolate in a medium saucepan over medium heat until chocolate has melted and mixture is well combined. Let frosting cool until creamy in texture.
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Run a knife around the edges of cake pans and invert cakes onto wire racks. Slice each cooled cake horizontally into three even layers. Set one layer on a rack set over a rimmed baking sheet. Pour some of the frosting over the first layer, smoothing with a spatula. Add a second layer and repeat process. Repeat process with remaining layers and frosting; smooth frosting over top and sides. Place remaining 6 tablespoons cocoa powder in a fine-mesh sieve; dust cake evenly with cocoa powder. Let cool completely before serving. Garnish with raspberries, mint, and whipped cream, if desired.