Preheat oven to 350 degrees F. In a large bowl cream the butter, peanut butter and sugars. Add the egg and mix well.
Add the flour, baking soda, and oatmeal and mix until well combined. Press into a 9×13 baking dish sprayed lightly with cooking spray. Mixture will be wet.
Bake for 12-15 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. The frosting makes a thin layer for a thicker frosting you can double the frosting.