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1
Preheat oven to 350 degrees F. Butter and 2 8-inch round cake pans (I use springform but any will do). Line with parchment paper, then butter and flour pans.
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2
Sift the dry ingredients into the bowl of an electric mixer. Mix on low speed for a couple minutes to incorporate. In a medium bowl, combine all buttermilk, oil, eggs, vanilla, zest, and Grand Marnier. Whisk to combine. With the mixer on low speed, slowly add wet ingredients. Once flour starts to disappear, add coffee, and continue mixing on low until batter is uniform.
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3
Split batter evenly between two prepared pans, and bake about 35-40 minutes, until a toothpick inserted in the middle of one of the cakes comes out clean. Cool cakes in pans for 30 minutes, then carefully transfer them to cooling racks.
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4
Place one layer on serving platter or cake stand. Using a long serrated knife, trim rounded part of top so top of cake is (relatively) flat. Flip upside down. Use about a third of frosting to frost what is now the top of that layer (using recipe below), then stack the second layer on top, right-side up. Use the rest of the frosting to coat the top of the second layer and the sides of the cake. Decorate as desired.
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5
Melt chocolates in double boiler. Remove from heat, let cool about 5 minutes, and whisk in sour cream and vanilla. Continue whisking to cool frosting to room temperature, and spread over cakes immediately. If making in advance, be sure to set frosting out on countertop at least an hour before spreading it, as it will turn hard in the refrigerator. If need be, frosting can be very gently reheated in double boiler over barely-simmering water just until spreadable, but be careful! Don't let frosting get too hot.
