I made this Over the Hill cake for my sister's 40th birthday, and it was actually quite easy! I made the cake from scratch, but any cake mix should suffice. I added black food coloring to the cake mix-probably about 3 or 4 drops.
My cake turned out purplish, but I used white cake, so you'll have to adjust to suit. After the cake was baked I let it cool completely-very important-and then shaved the top of the tombstone with a bread knife. Remember, you can always take more off, but it's a lot harder to put it back on.
Use a sawing motion when trimming the corners. I then mixed the icing (2 sticks butter + 4 cups powdered sugar + tsp vanilla + enough milk to make a smooth consistency) and added just enough black food coloring to give it a grey cast. Remember, when coloring icing the color will deepen as it sets, so go a couple shades lighter than what you're wanting. Place the cake on four squares of wax paper placed on the surface you will be leaving the cake on. This is so you can pull the corners of wax paper out after icing to keep the cake board or dish clean and neat.
Ice the entire cake with the grey icing. You can either leave the cake in one piece, or you can do like I did and crack it in the middle. If you want to crack it in the middle, then you need to use a cake board. Put you hand on the underside of the cake board and gently bend it toward the top middle of the cake. It should crack right open.
Remember it doesn't have to be perfect. Now take the rest of your icing and add more black food coloring to achieve a deep black. Then fill a pastry bag-or make a hillbilly pastry bag (a Ziploc with the corner cut off!) Using an appropriate size tip write your message on the cake. I wrote "Over the hill...but sexy still." Make sure to compensate for the crack-make it look like the writing was there before the crack. Do not do the writing before the cracking-it won't work! Just trust me. Change the tip and do the outlining and borders.
You can add candles if you want. We used black candles, and it was really cool. Hope you enjoy! Good luck!