Grease the bottom and sides of two 9 inch cake pans and line with parchment rounds in the bottom. Preheat oven to 350.
Beat butter, sugar and peanut butter a few minutes until fluffy.
Add sour cream and vanilla.
Add eggs, one a time to blend well.
Combine flour, baking powder and salt in a bowl.
Combine milk and water in a separate cup.
Add half the dry ingredients to the mix and blend. Add the liquids. Then add rest of the flour.
Blend well and pour into the two pans.
Bake 23-26 minutes or until a toothpick comes out with just a few crumbs attached.
Cool a few minutes and then carefully flip out onto cooling racks. I place a piece of parchment on the rack to prevent sticking.
Once cool, wrap each cake in foil and freeze until firm before assembling.