Box cakes are moist anyway, and I don't think adding buttermilk gives it any added moisture. The buttermilk is used to give it more of a "made from scratch" taste. To doctor a box devils food cake with buttermilk, use 1 1/3 c. buttermilk instead of the water. Add the other ingredients as directed. I also add a teaspoon of vanilla and 2 to 3 TBSP unsweetened cocoa. I used this for my daughter's birthday last month, and it tasted great. In my opinion, buttermilk doesn't work well with a box white cake.