They were a red velvet box mix and I used milk instead of water. They're perfect! (Thanks for the tip, Amber!)
I got the buttercream frosting recipe from a Kitchenaid cook book I have.
1/3 C softened butter
1/4 C whipping cream or evaporated milk
1 t vanilla
1/4 t salt
4 C powdered sugar
That's it! I used the evaporated milk, just to try it. It turned out delicious! I added blue food coloring to the mix once it was almost creamed to perfection.