How to Make It
In a large heatproof bowl set over a large saucepan of simmering water, combine the bittersweet chocolate, heavy cream, butter and sugar. Whisk occasionally until the sugar dissolves and the frosting is smooth. Transfer the frosting to a large bowl, cover and let stand at room temperature until firmed up, about 5 hours or refrigerate for up to 2 days. Let return to room temperature before frosting the cake.
Preheat the oven to 350°. Butter four 9-inch cake pans and line the bottoms with parchment paper. In a large bowl, sift together the flour, cocoa powder, sugar, baking soda, baking powder and salt. In another bowl, whisk together the buttermilk and eggs. Beat the buttermilk mixture into the dry ingredients until combined. Beat in the sour cream. Add the butter and grated beets and mix until fully incorporated.
Pour the batter into the prepared pans and bake for about 35 minutes, or until a tester inserted in the center of each cake comes out clean. Transfer the cakes to racks and let cool completely in their pans.
Invert the cake layers onto a work surface and peel off the paper. Set one layer on a large plate and spread 1/4 inch of chocolate frosting on top. Stack and frost the remaining cake layers. Spread the remaining frosting all around the sides of the cake. Cut the cake into wedges and serve.