Peanut Butter Frosting

I have been asked for the recipe for the peanut butter frosting I put on my Dad's birthday cake a couple of weeks ago. Here it is:

3/4 cup peanut butter (can also use soy butter if peanut allergies in family) 
1/2 cup water
1 tsp vanilla
4 cups powdered sugar

Cream peanut butter and vanilla. Add 4 cups powdered sugar and beat until combined. Add 1/2 cup water and beat until light and fluffy. (You may have to add in a little bit more water to reach spreading consistency depending on how oily your peanut butter is.)

My Dad's favorite cake to put this on is chocolate.