Preheat the oven to 350 degrees F. Grease a 13 by 9 by 2-inch rectangle baking pan with 1 tablespoon of the butter. Using an electric mixer, fitted with a wire whip, cream the remaining butter and sugar, scraping the sides down occasionally. Add the sour cream and mix to incorporate. Add the eggs, one at a time, beating well after each egg. Sift the flour, cornstarch and salt. Add the flour mixture, 1/2 cup at a time, scraping down the sides occasionally. Add the melted chocolate and vanilla and mix well. Dissolve the coffee in the water and add this mixture to the chocolate mixture, mix well. Pour the batter in the prepared pan. Bake the cake for 35 to 40 minutes or until the center is set. Remove from the oven and cool completely. Frost with Fudge Cake Icing.