1 package Devil's Food cake mix, 1/2 cup butter, softened, 1/2 cup shortening, 1 1/4 cups sugar, 3/4 cup milk, 1 t. vanilla extract.
Glaze: 1 cup sugar, 1/3 cup baking cocoa, 3 T. cornstarch, 1 cup water, 3 T. butter, cubed, 1 t. vanilla extract.
Prepare and bake cake according to package directions, using a greased and floured 13" x 9" x 2" baking pan. Cool for 10 minutes before inverting onto a wire rack. Cool completely.
For filling, in a mixing bowl, cream the butter, shortening and sugar until light and fluffy. In a small saucepan, heat milk to 140 degrees; add to the creamed mixture. Beat until sugar is dissolved. Stir in vanilla. Split cake into two horizontal layers; spread filling over bottom cake layer. Top with remaining cake layer.
In a saucepan, combine the sugar, cocoa, cornstarch and water until smooth. Bring to a boil over medium heat, stirring constantly. Cook and stir for 1 - 2 minutes or until thickened. Remove from the heat; stir in butter and vanilla until glaze is smooth. Cool to lukewarm. Spread over top of cake. Let stand until set. Refrigerate leftovers.