In a large mixing bowl or bowl of a stand mixer, cream together the softened butter, 1 cup of peanut butter, sugar, and brown sugar. Mix in the eggs and vanilla.
Add the oats, flour, baking soda, and salt to the butter mixture and stir to combine.
Spray a jelly roll pan with nonstick cooking spray and press the dough evenly, covering the bottom of the pan. Bake the cookie at 350 degrees for 15 minutes.
Remove from the oven and drop the remaining peanut butter by teaspoons over the cookie layer. When the peanut butter begins to melt, carefully spread over the cookie.
When the peanut butter has set up, mix together the frosting ingredients until smooth and use an offset spatula to spread over the peanut butter layer. Cut into bars and serve. Tip: Refrigerate the bars before spreading the frosting and before cutting into bars for best results.