Preheat oven to 345°F. Lightly grease two 8-inch layer cake pans with vegetable oil or butter, or line with parchment and flour (recommended).

Melt chocolate and let cool. Sift flour, sugar, baking soda and salt together in a mixing bowl. In a separate measuring cup, blend the hot coffee, sour cream and vegetable oil.

With the mixer on low speed, add the coffee mixture in a stream to the dry ingredients and mix until blended, about 35 seconds. Stop the mixer several times to scrape the sides.

Add the eggs one at a time and mix on medium low speed after each addition until smooth, about 15 seconds. Scrape the bowl each time. Add the chocolate and mix until the batter is uniform in color.

Divide the batter evenly between the prepared pans and place them on the center rack of the oven. Bake until the cake springs back to the touch and a tester inserted in the center comes out clean, 35 to 38 minutes. Cool before frosting.

Place all ingredients except vanilla in the top of a double boiler placed over simmering water and cook uncovered until the chocolate is melted and the sugar is dissolved, 30 minutes. The mixture will be smooth and velvety. Pour the fudge into a small bowl, stir in vanilla, and refrigerate until thick and of spreading consistency, about 2 hours.

Add heavy whipping cream to mixer bowl fitted with whisk attachment. Beat for a minute, then add powdered sugar. Continue to beat for several minutes until the whipped cream is stiff and holds peaks.

Split cake layers in half each. Spread the fudge filling and whipped cream as follows: cake layer, ¼ cup fudge, ½ cup whipped cream (extend just beyond the edge of the cake), cake layer, ¼ cup fudge, etc. Top with remaining whipped cream. Press down each cake layer so that the whipped cream is squeezed out a little.