Line a 9 inch round cake pan with parchment on the bottom and grease the bottom and sides for easy removal. In a large microwavable bowl, heat marshmallows and butter or oil for 2 minutes, stir and heat again in 1 minute increments until smooth. Remove from microwave and stir cereal into the marshmallow mix. Pour into cake pan and let cool and set. Then remove and place on serving plate. Do not cut a hole in the center of the cereal layer.

Line two 9 inch round cake pans with parchment paper and grease well for easy removal. Mix and bake cakes according to package instructions. Let cool and remove from pans. Set aside on a clean work surface.

Level tops of cakes and cut a 3 inch circle out of the middle of both of the cakes. Reserve the circle cut out of the middle of one of the cakes.

Spread a thin layer of frosting on top of the cereal layer. This will help the cake stay in place and keep the slime from oozing out. Stack one layer of the cake on top of the cereal layer. Put your frosting or filling on the cake layer. Place the second layer of cake on top of the first layer.

Within about two hours prior to cutting the cake, pour the can of pie filling into the hole in the middle of the cake leaving enough room for the round piece of cake you reserved to "plug" the hole. Place the reserved cake in the hole. The longer you let the cake sit before cutting into it with the filling in the middle, you will risk a little filling sneaking out (especially if you have a very runny filling). The cereal layer and the frosting between the cereal layer and the first cake layer will help prevent this from happening, but I wouldn't push the timing too much.

Frost and decorate the cake as you wish. I cut strawberries and decorated the top of the cake to disguise any evidence of the hole in the center. When you are ready to cut your cake, be sure to cut all the way to the center and take out a large enough slice (about 3 inches) to allow the slime to pour out. It looks awesome when it does and the kids seriously love it! I thought it was pretty cool too;-)