Combine the cake mix, eggs, water and butter as directed on the box. Pour batter into sprayed/greased 9" round cake pans.
Bake 30-35 minutes or until a toothpick inserted comes out clean. Cool on wire rack for one hour or more.
Using a cutter (or any other genius tools you have), remove the center of your bottom layer. Eat this part :)
Smooth on the frosting on the top and inside the hole. Pour in the M&M's.
Place the other cake on top.
Add frosting to the sides and top. Finish off with your decoration of choice. Serve!
Notes
ITEMS/INGREDIENTS USED IN RECIPE - If you are interested in any of the items I may have used (or are similar to the items I used) in making this recipe, I have listed all or some of them above within the post. NOTE: These links are affiliate links of which I receive a small commission from the sale of certain item. This does not affect your price, it remains the same.