Preheat oven to 350°F; line two 9-inch cake pans with parchment paper and lightly butter.

Beat butter, shortening, and sugars together until smooth. Beat in the eggs, one at a time, until smooth and light.

Stir the buttermilk and vanilla together. Sift the flour, cocoa, baking powder, baking soda, and spices together in a large bowl.

Alternately stir the flour mixture and buttermilk, in thirds, into the beaten butter. Blend well with each addition.

Pour batter into the pans and bake until center tests clean -- about 35 minutes. Cool cakes in the pans on a rack for 30 minutes, unmold, and cool completely before frosting (if desired).