For the bars: In a stand mixer, cream butter with sugars until light & fluffy. Add vanilla & eggs & blend. Add 1 cup of peanut butter & mix thoroughly. Scrape bowl.
In a separate bowl combine the flour, oats, baking soda & salt. Slowly add to moist ingredients in mixer. Mix until it just comes together. Make sure to scrape the bottom & mix again. The mixture should be moist, but slightly crumbly.
Press into a 10x15" jelly-roll pan, can use a rolling pin to even everything out (might need to lightly flour it first.)
Bake at 350 degrees Fahrenheit for 12-15 minutes, just long enough to cook it. Do NOT over-bake or you will dry it out. Let rest 3 minutes.
While bars are still warm, spread a layer of peanut butter over the top (about 1 cup). Let the peanut butter dry on top; can speed up the process by placing the bars in the fridge.
Mix frosting: combine butter, vanilla, milk, cocoa, & powdered sugar in a bowl with a hand mixer. Blend until thick, creamy, & smooth. Spread frosting over the top of the peanut butter layer of your bars. Cut bars & serve.