Cherrybrook kitchen makes an amazing yellow and chocolate cake mix that is dairy, egg, and nut free. They are available at whole foods, jewel, and sunset markets. They are usually found in the organic ailse at jewel. The yellow cake mix calls for dairy free margerine; the only two dairy free margerines are mothers and earth balance.
I also have a chocolate cake recipe that is awesome and dairy/egg free. The recipe is:
mix dry ingredients in a bowl: 1 1/2 cups flour; 1 cup sugar; 1/4 cup baking cocoa; 1 teaspoon baking soda; 1/2 teaspoon salt
add the following to the dry ingredients and mix vigorously by hand about one minute or until well blended: 1/3 cup vegetable oil; 1 teaspoon white vinegar; 1/2 teaspoon vanilla, and 1 cup cold water.
Pour into cupcake liners (makes 12-13) or a greased/floured 8" cake pan. Bake at 350 for about. 20 min for cupcakes and 30 for cake. Test for doneness with a toothpick.
I also add chocolate chips when I have them; choose bakers or president's choice "the decadent" as they are both dairy free. However, I do believe bakers processes in a facility shard with milk.
As for frosting, I use pilsbury canned in a pinch or I cream 1/2 crisco with 1/2 cup margerine. Then I add 4 cups powdered sugar. Next I mix 1 tablespoon vanilla and about 2 tablespoons soy or rice milk. Add enough s/r milk to reach desired consistency.
Hershey's cocoa also has a great choc frosting recipe on the back or their cocoa; just sub margerine/soy or rice milk in the recipe.