It is actually Chocolate cake with red food coloring. It isn't just for Groom's cake. A bakery birthday cake (in Texas) will be Chocolate, White or Red.
This is the recipe I use which gets rave reviews. It is from the book the "Cake Doctor", it is as good as the best bakery in town. I sometimes make it without food coloring, and call it "brown velvet". It is a color of Milk Chocolate that is a little too old.
1 package white cake mix with pudding (sifted)
1 cup buttermilk
1 stick margarine
3 tablespoons Cocoa powder
3 large eggs
1 or 2 bottles food coloring (the original recipe had two, but one is still very red, and when food coloring reached $2.80 a bottle, I cut back)
1 teaspoon Vanilla extract
Cream Cheese Frosting
Preheat oven to 350
Use 2 round cake pans, greased and floured
Put in everything but the food coloring
Blend on low for 1 minute
Blend on med for 2 minutes
Add food coloring and blend (Itís less messy of you add the food coloring last)
Bake at 350 until they spring back, about 28-30 minutes
Hint to keep the cake mostly flat. Tear a terry cloth towel into one long strip, by tearing almost down to the other edge, then starting at the other side tear almost down the first edge. Soak and wring the towel strip. Wrap it around the edge of the cake pan. This will soak up some of the heat, and keep the edges from cooking too fast. Also, put a bowl of water in the oven.
Cream Cheese frosting: (from memory)
1 8oz package cream cheese at room temperature.
1/2 stick (4oz.) Unsalted butter at room temperature
1 bag confectioner's sugar (about 4-5 cups)
a couple of tablespoons of vanilla
Cream the cheese and butter on low speed in a mixer. Slowly add the sugar using a sifter until it is very thick. Won't be all of the bag, but most. Add the vanilla. continue stirring on low. When mixed increase to medium high for a couple of minutes.