I just adapted this recipe for my son's birthday. The original recipe is Chocolate Buttermilk Cake from The Fannie Farmer Cookbook. I've adapted it to be gluten-, soy- and dairy-free (it already was egg free). I only made it as cupcakes, but it was awesome!
1 2/3 cups flour (I used Bette Hagman's 4 flour bean mix to make it gluten-free)
1 cup sugar
1/2 cup unsweetened cocoa
1 tsp baking soda
1/2 tsp salt
1 cup buttermilk or sour milk (I used 1 Tblsp. vinegar + hemp milk to make 1 cup)
1/2 cup vegetable oil (used 1/4 c. canola oil & 1/4 c. applesauce)
2 tsp vanilla
Preheat oven to 350 degrees. Butter and lightly flour two 8-inch round cake pans or one 9 x 13-inch pan. Mix the flour, sugar, cocoa, baking soda, and salt in a bowl. Add the sour milk, vegetable oil (and applesauce), and vanilla, beating until smooth. Spread in the pans or pan and bake, about 20-25 minutes for the small pans, 35-45 minutes for the large one, (23 minutes for cupcakes). Test to see if a toothpick comes out clean. Cool for 5 minutes in the pan before turning out onto a rack.