Twinkie Cake

1 (18.25 ounce) box yellow cake mix
1 cup milk or eggnog
5 tablespoons flour
1 cup (2 sticks) butter, softened
1/2 cup white Crisco
1/2 teaspoon salt
3/4 cup granulated sugar

Bake cake mix as directed on package, using 9-inch round or square pans. Let cool and slice each layer in half horizontally to make 4 layers. Cook milk and flour until thick; cool, then add remaining ingredients. Beat until fluffy. Use mixture to frost between layers but not sides.