For my fiances 30th birthday he requested a cake that had mint and chocolate which didn't come as a surprise to me since he has made this request before (see the mint Oreo cake from birthday's past here). I basically made the same thing with the ingredients all switched around (only mint in the icing not the cake) but he seemed equally pleased with the end result.
1 box chocolate cake (plus eggs/water/oil it calls for)
1 package of Oreo cookies
1.5 sticks of butter (12 oz)
1 package of cream cheese
3.5 cups confectioners sugar
1 cup milk chocolate chips
2 tablespoons peppermint extract
1 teaspoon vanilla
pinch of salt
green food coloring (optional)
1. Prepare boxed cake batter according to instructions. Stir chocolate chips into batter and bake according to instructions on box. I used two 9 inch cake pans.
2. While cake is baking, take one sleeve of Oreos and break up into a food processor. Blend them into fine crumbs while adding in 1/2 stick (4 oz) melted butter to make them sticky. Set mixture aside for assembly later.
3. Also while cake is baking whip together 1 stick of butter with cream cheese until fluffy. Add in vanilla, peppermint extract, and salt then gradually start adding in sugar 1/2 cup at a time until all 3.5 cups is incorporated (I also used a few drops of green dye here but it is not necessary).
4 Assembly: Place one cake on cake stand and ice with mint frosting, then add crumbles very generously to the top, then add the other cake on top of the crumbles and ice all around and use up the rest of the crumbles for the topping.