Bake 1 yellow cake mix in 9 x 13 inch pan. Cool. Split lengthwise (I use a jelly roll pan lined with wax paper and cut cake in half. Wax paper will peel off.)
Cook until thick: 5 tablespoons flour and 1 cup milk, stirring constantly. Cool.
Beat until fluffy: 1/2 cup Crisco, 1/2 cup butter, 1/2 teaspoon salt, 1 teaspoon vanilla, and 1 cup sugar.
Stir flour mixture well. Slowly add to sugar. Beat until very fluffy. Use a small, deep bowl.
Put filling between layers of cake and dust with powdered sugar. Refrigerate. Also good with chocolate cake for Suzy Q's.
