If you're wiling to try to make it yourself, this is a *really* great recipe. I've made these several times, and they are delicious! Note that the recipe only makes 12 cupcakes, so you will need to double it if you want 24 cupcakes or 2 layer cakes. The icing is really great too!
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Re: Vegan Birthday cakes
https://dulcevegan.com/orders/
they are pretty darn tasty!
Dough Bakery in Inman Park also makes vegan cakes: https://doughbakery.com/