Preheat oven to 350F. Grease and flour a 10x15x1-inch jellyroll pan. Combine all cake ingredients. Beat until smooth. Pour into prepared jellyroll pan. Bake 15 minutes. Cool.
Whipped Cream Filling: Bring cornstarch and milk to a boil, stirring constantly, until very thick. Let cool completely. Cream together butter, Crisco, sugar and vanilla. Add the cooled mixture and beat at high speed until fluffy. Spread over cooled cake.
Chocolate Frosting: Melt the butter, and mix in remaining ingredients until smooth. Pour over Whipped Cream Filling. Refrigerate until frosting is set.