My niece is having her first birthday in December. She suffers from severe food allergies that cause gastric bleeding, projectile vomiting, and hives. She is on a very restricted diet. My sister wants to make her a smash cake for her party but it needs to be free from all major allergens.
We desperately need help finding a recipe for a wheat, gluten, dairy, egg, and soy free cake.
Any help would be greatly appreciated.

We make gluten/dairy/egg/soy-free cakes all the time. Our recipe is proprietary, but it is based on our gluten-free flour mix (rice, sorghum, tapioca, and xanthan gum) and ground flax seeds, with palm oil as the base for the frosting.

We're working on putting together a recipe book, but in the meantime you might want to check out this one by Elizabeth Gordon:
http://amzn.to/pV1rzi

I just happen to come across this. I would think it could easily be turned into a cake instead of cupcakes. Hope this helps!

http://blog.thecelebrationshoppe.com/2010/10/25/allergy-free-halloween-treats/

No eggs, gluten, dairy, or soy

WACKY CAKE
1 1/2 cups GF flour blend*
1 tsp baking soda
1/2 tsp salt
1 cup sugar
3 tbsp unsweetened cocoa powder
1 tsp vanilla
1 tsp vinegar
5 tbsp oil
1 cup water
Preheat oven to 350 degrees F. 8" round pan lined with paper.
In a large mixing bowl, mix GF flour, sugar, cocoa powder, baking soda and salt.
Mix water, oil, vinegar, and vanilla in a bowl or jug. Pour over the dry ingredients, and stir until mixed. BATTER WILL BE THIN.
Bake 25-28 minutes. Check if done before removing.
Cake will be fragile while hot: Stand 10 minutes before turning out ..
* select one that includes xanthan gum or guar gum

Simple Buttercream
2 cups confectioner's sugar [check if GF]
4 oz dairy-free spread eg nuttelex
1-2 ts vanilla extract
Cream the spread in the bowl of a stand mixer.
Add the pinch of salt and then sugar, a bit at a time.
Add vanilla.[or extract of choice].
Thin to spreading consistency with the liquid of your choice.

I haven't tried this but what about:

http://www.nourishingmeals.com/2009/02/gluten-free-vegan-sugar-free-chocolate.html