Preheat oven to 350 degrees. Grease two 8 inch round cake pans and line the bottoms with wax paper or parchment circles. Grease the paper and dust the pans with flour.
In a medium bowl, sift together the flour, cocoa, baking soda, and salt.
With an electric mixer, beat the butter and sugars on high speed until light, approx. 5 minutes. Add the eggs and vanilla and mix well, scraping bowl often.
Combine the buttermilk, sour cream, and coffee in a small bowl or large Pyrex mixing cup. On low speed, add the flour mixture and the buttermilk mixture alternately in thirds, beginning with the buttermilk mixture and ending with the flour mixture. Mix the batter only until blended.
Divide the batter between the two pans and smooth the tops with a spatula.
Bake on the middle rack for 25 to 30 minutes, or until a toothpick comes out clean. Cool for 10 minutes on a rack, remove from the pans, and allow to finish cooling.
Place cake on a serving plate, flat side up. Frost the top of that layer with the butter cream (I use the frosting below). Place the second layer on top, also flat side up, and front the top and sides.
To make the frosting, place chocolate in heatproof bowl. Bring cream to boil in small saucepan over medium-high heat; pour over chocolate. Add corn syrup and let stand 3 minutes. Whisk gently until smooth; stir in vanilla. Refrigerate 1 to 1 1/2 hours, stirring every 15 minutes, until mixture reaches spreadable consistency.