Preheat oven to 350°.
Cream together butter, peanut butter and sugars until light and fluffy with either a stand mixer or electric hand mixer.
Add in the eggs and vanilla, mix until just combined.Add in the flour, old fashioned oats, salt and baking soda.
Mix until the cookie dough forms. Dough will be slightly sticky.
Spread and press the dough into the bottom of a greased 9x13 pan.
Bake at 350° for 18-20 minutes.
Once the peanut butter bars come out of the oven, make the frosting.
To make the frosting, melt the butter, cocoa powder and milk in a medium saucepan pan over medium heat. Bring to a soft boil.
Once boiling, remove from heat and whisk in the powdered sugar and vanilla.
Let the icing cool for 10 minutes. Stir the icing to break up the crusting on top and then pour over the peanut butter bars.
Allow to cool completely before cutting. (can put in fridge to speed up the process if desired.)