Prepare and bake the cake according to package directions, using a greased and floured 13 X 9 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 the 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 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 heat, stir in butter and vanilla until glaze is smooth. Cool to lukewarm. Spread over top of cake. Let stand until set. Refrigerate leftovers.