5To assemble, spread the filling between the cooled cake layers and top with the frosting.
In a 1-quart saucepan, melt 3 tablespoons butter and the chocolate over low heat, stirring occasionally.
Meanwhile, in 1-cup glass measuring cup, microwave the water uncovered on High 15 to 30 seconds or until hot. Remove chocolate mixture from heat. Stir in the powdered sugar and 3/4 teaspoon vanilla. Stir in 3 tablespoons hot water. Stir in additional water, 1 teaspoon at a time, until icing is smooth and thin enough to spread.
I want to add a note here..if you are unfamiliar with this frosting, it is a strong bittersweet chocolate flavor..the classical frosting for on this type of cake. I am thinking you could add more sugar, and then thin down some more with a bit more water, to get a sweeter frosting. I like it the way it is, but have just a sliver, as it is rich..Sometimes my Gram would just use a store bought frosting, heat it up in the microwave, and pour it on..there are many options.