Bake 2 square cakes, using 10"x10" cake pans and following recipe/cake mix instructions.
While the cakes are baking, mix up both the regular buttercream frosting and the chocolate buttercream.
The regular buttercream icing needs to be colored green, so add enough green food color until satisfied with the shade you have.
When the cakes have cooled, place one cake on a 12-inch square cake board that's been wrapped in white freezer paper.
Optional but very helpful step at this point: Slice the rounded top off of the cakes with a cake slicer/leveller. This helps the cakes to have a flatter shape and makes it easier to decorate them.
Spread a portion of the chocolate buttercream frosting on top of that cake.
Then sandwich the two cakes together.
Make the rest of the chocolate buttercream frosting look more like dirt and mud by crushing up the Peanut Butter Oreos and mixing them in with the frosting.**
Spread the chocolate buttercream "dirt" frosting on the sides of the cake, working your way around.
Using Wilton Tip 233, add the grass with the green buttercream frosting. Make sure the icing is thin enough to easily squeeze through the grass tip, or it will be very difficult. The Wilton website gives a great tutorial for how to create the grass, but basically, just squeeze the pastry bag to create each little patch of grass, and then pull up or away.
When finished, decorate your masterpiece with candles or whatever your heart desires.