The Filling:

Cream the butter 5 minutes, medium speed. Add Crisco a little at a time. Cream another 3 or 4 minutes. Add the sugar a little at a time while continuing to beat. Then add the milk, mixed first with the vanilla, beating and scraping the sides and bottom of the bowl frequently. The longer you beat this the better it becomes.

The Cake:

Preheat oven to 350. Grease two 8-inch-square cake pans or baking dishes, or use a 13x9x2-inch pan. Beat 1 minute after each addition. Pour batter into prepared pans. Bake at 350 for 30-35 minutes (40-45 minutes for an oblong pan) or until you can insert a toothpick and it comes out clean. Cool the cake in pan on a rack. It's best to use the cake for Twinkles when it is slightly frozen - about 30 minutes in the freezer. Then cut them into 1-1/2 by 3-1/2 inch bars. Put bottom side of each bar up on waxed paper. Spread half of the bars with the filling and put together with an unfrosted bar, sandwich style. Refrigerates up to 2 weeks, freezes up to 1 year.