Preheat oven to 300°F. Spray three 9 inch round cake pans with flour-added cooking spray. Alternately, you may use two 10x2 inch pan. Line bottoms with rounds of parchment paper (this is a sticky cake).
Combine chocolate with hot coffee and stir until smooth. Let cool slightly while you prepare other ingredients.
In a large mixing bowl, mix together sugar, flour, cocoa powder, baking soda, baking powder, and salt.
In another mixing bowl, using a hand-held electric mixer, beat eggs for 3 to 5 minutes or until light and lemon-colored. Using a whisk or lowest speed of mixer, add oil, buttermilk, vanilla, and melted chocolate mixture.
Add flour mixture to egg mixture and beat on medium speed until blended. Use a heavy duty scraper as you mix and be sure to scrape bottom of the bowl.
Divide batter between pans and bake in middle of oven at 300 until a tester inserted in center comes out clean. Bake time for 9 inch pans is about 50 minutes. Bake time for 10 inch pans is 60 to 75 minutes.
Set the pans on a wire rack and let layers cool completely in the pan.
Loosen edges with a knife and invert layers onto racks. Carefully remove parchment paper and let cool completely.
At this point you can frost the chocolate layer cake or you can wrap layers tightly in plastic wrap and freeze until ready to use.
To make the frosting, In a large mixing bowl, use a heavy duty scraper to mix the softened butter, sour cream, vanilla, 1 cup of the confectioners’ sugar, cocoa, salt and about 2 tablespoons of the cream until thick and cohesive (this keeps the sugar from flying everywhere). When you have a big cohesive chocolate blob, beat with an electric mixer until smooth, gradually adding remaining sugar and more cream and buttermilk (or whole milk) as needed. Scrape bowl and continue beating until smooth and creamy. Spread over one of the cakes. Stack cakes and spread over the sides.