The birthday in question may have already taken place, but I'll still suggest Pam's Patio Kitchen (11826 Wurzbach at Lockhill Selma). I think their homemade layer cakes are delicious, and the red velvet always sells out particularly quickly. You'd need to call them to inquire about buying a whole one: (210) 492-1359.
FYI: Pam's is a restaurant, not a bakery. You can search this board for more reviews of their food.