Preheat oven to 350 degrees F and spray a 9×13 baking dish lightly with cooking spray.
In a large bowl cream the butter, peanut butter and sugars. Add the egg and mix well.
Add the flour, oats, baking soda, salt and and mix until well combined. Mixture will be wet. Press into prepared pan.
Bake for 15-18 minutes, or until the edges are golden brown. Remove from the oven and let cool completely before frosting.
FOR THE FROSTING: Combine the butter, peanut butter, and vanilla in a medium sized bowl and beat until fluffy. Add the powdered sugar and add the milk gradually until you reach the desired consistency. You may not need it all. Frost cooled bars.