Happy birthday to your son!
If you would like to use a mix, try Whole Foods or Sprouts/Henry's.
Here's a recipe for a vegan chocolate cake and frosting:
http://www.veganrepresent.com/forums/showthread.php?t=2772
Vegan Chocolate Cake With Creamy Chocolate Frosting
(Preparation 3 hours, 1 hour active)
For the cake:
1 2/3 cups natural large-crystal cane sugar (testers preferred Sugar in the Raw)
2 1/2 cups all-purpose flour
1/2 cup oat flour
2 teaspoons baking soda
3/4 teaspoon salt
I cup Dutch-processed cocoa (testers preferred Scharffen Berger Natural Cocoa Powder)
I cup natural cocoa
1 1/2 ounces unsweetened chocolate, chopped
1 cup hot brewed coffee
1 cup light coconut milk
2 tablespoons cider or white wine vinegar
1 1/2 teaspoons vanilla extract
3/4 cup vegan butter substitute (testers used Spectrum Spread)
For the frosting:
Two 10-ounce packages vegan or other semisweet chocolate chips (testers used Tropical Source)
1/2 cup hot brewed coffee
4 tablespoons boiling water
1/2 cup light coconut milk
4 ounces silken tofu
To make the cake:
Adjust oven rack to middle position and heat oven to 350 F. Spray two 9-inch round cake pans with nonstick cooking spray; set aside.
Process sugar in a food processor to a fine powder, 30 to 40 seconds. Sift sugar, flours, baking soda and salt into a large bowl, then whisk to combine.
Combine cocoas and chocolate in a large bowl and cover with hot coffee; whisk until smooth.
Combine coconut milk, vinegar and vanilla in large measuring cup. Place vegan butter substitute in a medium bowl. Add coconut milk mixture in two additions, whisking until smooth after each.
Add vegan butter-coconut milk mixture to the chocolate mixture and whisk to combine, then add this mixture to dry ingredients and fold gently with rubber spatula until just incorporated and no streaks of flour remain.
Divide batter evenly between cake pans and bake until toothpick inserted in center comes out clean, 20 to 25 minutes, switching position of and rotating pans after 12 minutes. Cool cakes in pans on wire rack to room temperature, about 2 hours.
To make the frosting:
Place chocolate chips in a medium bowl set over a saucepan of barely simmering water. Pour hot coffee and boiling water over chips, then whisk until smooth. Whisk in coconut milk until incorporated.
Process chocolate mixture and tofu in food processor until smooth and combined, 10 to 15 seconds, scraping down bowl once or twice.
Transfer the mixture to a medium bowl and cover with plastic wrap. Refrigerate until cool and texture resembles firm cream cheese, 1 to 1 1/2 hours. (If mixture has chilled for longer and is very stiff, let stand at room temperature for 1 hour.)
Transfer cooled chocolate mixture to the bowl of standing mixer fitted with whisk attachment. Whip mixture at high speed until fluffy, mousse-like, and the mixture forms medium stiff peaks, 1 to 1 1/2 minutes.
To assemble, invert cakes from their pans. Spread a dab of frosting in center of a cardboard round cut slightly larger than a cake. Place one cake, centered, on the cardboard round. The frosting helps hold it in place.
Using an icing spatula, spread about 1 cup frosting evenly onto the top of the first cake. Place the second cake on top of frosted bottom layer and spread about 1 cup frosting on top. Cover sides of cake with remaining frosting.