Special equipment: 10-inch springform pan 21/2-inch wide acetate ribbon, see Cook's Note*.
Preheat the oven to 325 degrees F.
Butter and flour a 10-inch springform pan.
Sift the flour, cocoa, baking soda, and salt 3 times, set aside. Cream the butter with the sugar until light and fluffy. Add the eggs 1 at a time beating well after each addition.
Combine the buttermilk, coffee and vanilla extract.
Mix in 1/3 of the dry ingredients, then half the wet ingredients. Repeat with the remaining dry and wet ingredients, finishing with dry. Pour the batter into the prepared cake pan. Bake for 1 hour, until a skewer inserted in the middle of the cake comes out clean. remove the cake from the pan after 15 minutes. Cool completely. Cut the cake into 2 layers.
Sandwich whipped ganache between the 2 layers and spread more ganache on the top and sides of the cake. Wrap the chocolate covered acetate around the cake, chocolate side in, overlapping the ends slightly. Refrigerate the cake until the chocolate in the ribbon has set. Carefully remove the acetate before serving.
In a medium pot over high heat bring the cream to a boil. Remove from the heat and stir in the chocolate. Stir rapidly until all the chocolate has melted and the mixture is a smooth even color. Cool to room temperature stirring occasionally. Cover and refrigerate overnight. Whip the ganache in a chilled bowl until stiff peaks form, see Cook's Note*.
Cut a piece of 2 1/2-inch wide acetate ribbon long enough to wrap around the cake completely. Lay the acetate on a flat surface and spread the chocolate evenly over the entire surface using an offset spatula.
Carefully lift the acetate, chocolate side towards the cake, and wrap it around, lightly pressing it to the side of the cake. Work quickly; the chocolate should still be sticky when wrapped around on the cake. Refrigerate the cake until the chocolate in the ribbon has set.
Do not over whip as the ganache may split. Acetate ribbon is a medium gauge clear plastic often used for cake decorating and chocolate work. Sold in rolls, it can be found in baking supply stores.