Prepare Icing 24 hours ahead of time if possible
Cream together margarine, shortening, and butter, and vanilla
Add powdered sugar, then Peanut Butter. Mix 3 to 5 minutes on medium speed
If you are making it ahead of time, cover it with plastic wrap and leave it at room temperature
For the cake, separate the wet and dry ingredients into two different bowls
Mix the wet ingredients with a whisk, then slowly add the dry ingredients
If you are using a mixer blend on a slow speed
Once the butter is thoroughly incorporated, divide evenly and pour into two 9" pans that are greased with non-stick release spray and dusted with flour.
Bake at 350° for 22-25 minutes
Once cakes have cooled completely, cut layers in half and put the icing in between all layers
I made this a three-layer cake, but you can make it a four-layer cake if you want
Ice the outside of the cake with a thin layer of icing first and place in the refrigerator for about 20 minutes to set the crumbs
Then ice the cake with a finishing layer of icing
Place Peanut Butter cups on the cake for decoration if desired