Preheat oven to 325 degrees F. Lightly coat 2 (8-inch) nonstick pans with cooking spray.

In a large bowl with an electric mixer, beat together the cake mix, oil, eggs, and water. (If mixing by hand, beat vigorously for 2 minutes.)

Divide batter evenly among the prepared pans. Bake in the oven for 30-35 minutes until a toothpick inserted in the center comes out clean. Cool 10 minutes before removing from pans. Cool completely before proceeding.

Cut a 3 1/2-inch hole from the center of one cake layer. Set aside.

Score a 3 1/2-inch circle in the center of the other cake layer, then scoop out the middle without going all the way to the bottom. Transfer this layer to a serving plate. Spread 1/2 cup of the frosting on the scooped-out layer cake (but not inside the hole.) Place the first cake layer on top.

Pour the M&Ms inside the hole.

Take the reserved 3 1/2-inch cut-out from the first layer and slice off half of it to make a thin disc; place on top of the M&Ms to form a lid.

Frost the top and sides of the cake with the remaining frosting. Decorate the top with sprinkles of your choice.

Refrigerate until ready to eat. Make slices not quite to the middle of the cake and watch the candies tumble out. Fun!