I made this cake two years ago for a Halloween party. Caution: Sugar-coma in a bowl.

Dirt Cake Recipe
2 packages Oreos (pulverize them in a blender)
1/2 stick butter
8 oz. block of cream cheese
1 cup powdered sugar
3 1/2 cups milk
2 small boxes vanilla instant pudding mix
12 oz. tub of cool whip
1 bag of gummi worms
-Cream the butter, cream cheese, and powdered sugar.
-Mix pudding packets with milk and then add the cool whip.
-Combine pudding mix with cream cheese mixture.
-Some recipes tell you to layer the cookies and the pudding but at this point I go ahead and mix in about half of the cookie crumbs along with most of the gummi worms.
-Use the rest of the cookie crumbs to cover the top of your cake and a have a few gummi worms sticking out of the dirt as well.
-Keep in refrigerator until it is ready to serve (I like to make it the day before a party)
Tombstone Sugar Cookies
Make your favorite sugar cookie dough (or buy one of those refrigerated packages). Now the trick to getting the marbled effect on the cookie is to add the tiniest bit of black cake icing coloring (not regular food coloring). They come in a little bitty jar. Stick a toothpick into the color and wipe it on the dough, then work the dough with your hands until it is nice and marbled. Don't over work it or it will be some icky grey color. Roll out your dough and use tombstone and coffin cookie cutters to punch out your shapes. Bake as per directions of recipe or package. The ghosts are made from un-marbled cookie dough. I used gel icing tubes to decorate my cookies.
The cookie cutters and gel icing came from Micheal's.
The cake icing coloring is from a specialty cake supply shop, but can be ordered online and in found in some craft and hobby stores.
Now when you pull out your dirt cake just strategically place some tombstones and coffins on top and serve to your very impressed guests. Or the cookies can be served by themselves with guests names written on them in icing.