1.5tbspmilk. Amount is "more or less" depending on consistency you need.
1tspVanilla extract (optional)
Instructions
beat peanut butter and butter in bowl until well-combined. It's important that the butter be at room temperature.
Blend in the sugar, a quarter cup at a time, until well mixed
Blend in the vanilla extract, if using.
Add the milk, 1/2 tbsp at a time, until the frosting is spreadable. Don't add too much, since you don't want the frosting oozing out between the layers.
Notes
This is more or less a standard recipe for butter cream frosting, but with peanut butter replacing some of the butter.