Preheat oven to 350 degrees F. Spray a large (17 x 12 x 1 inch) baking sheet with non-stick spray. Set aside.In a large bowl, beat butter, sugar and brown sugar together with an electric hand-mixer until combined and creamy.Add peanut butter, eggs and vanilla extract. Mix well to combine.
Slowly mix in flour, oats, baking powder and salt. Mix until well combined.Spread cookie dough out in an even layer on the prepared baking sheet. (This cookie dough is sticky so this takes a little patience to spread out. I spray the back of a spoon with cooking spray to help spread the batter out.)Bake bars for 16 to 18 minutes. They will be nice and golden. (I bake mine for 18 minutes in my oven.) Remove bars from oven and let cool.
FROSTING: Mix all frosting ingredients until smooth and creamy.Spread frosting evenly on top of peanut butter bars. Cut into squares. Serve and enjoy!