Preheat oven to 350 degrees. Line three 8-inch cake pans with parchment paper to keep cakes from sticking to pans.
Melt unsweetened chocolate, taking care not to burn the chocolate. Mix with cocoa, butter, and sugar. Combine well, then beat in eggs. Add coffee, yogurt or sour cream, and vegetable oil.
Stir flour, baking soda, and salt together. Mix dry ingredients into the chocolate mixture, just until no flour streaks remain. Divide batter among three pans. Bake about 35 minutes, or until a wooden pick inserted in the center comes out clean. Do not wait for crust to form around the edges.
Ice the cake with your favorite frosting. I often use Julia Childs' recipe for uncooked butter cream or Italian meringue butter cream, flavored with Grand Marnier liqueur and grated orange zest, or with dark or white chocolate.