Preheat oven to 350℉ (177℃). Spray a half-sheet baking sheet (13"x18") with baking spray.
Beat the butter, granulated sugar, brown sugar, and 1 cup of the peanut butter in a mixer until creamy and light. Add eggs and vanilla extract, then beat again, scraping the bottom and sides of the bowl.
1 cup (227g) salted butter, 1 cup (200g) granulated sugar, 1 cup (200g) packed light brown sugar, 2 large eggs, 2 teaspoons vanilla extract, 1 cups creamy peanut butter
Add the flour, oats, baking soda, and salt. Mix just until combined. Spread the peanut butter oatmeal cookie dough into the prepared pan with a spatula. Bake for 15-17 minutes until golden brown around the edges and barely set in the middle. Do not overbake.
2 cups (282g) all-purpose flour, 2 cups old-fashioned oats, 1 teaspoon baking soda, 1 teaspoon salt
While the peanut butter oatmeal cookie base is still hot, melt the remaining 1 1/2 cups of peanut butter in the microwave in a microwave safe bowl for 1 minute, then drizzle over the top of the bars, spreading it into an even layer with a spatula or knife. Let cool completely until the peanut butter has firmed up. This can be sped up by sticking the bars in the fridge.
1.5 cups creamy peanut butter
Make the chocolate frosting by beating the butter, powdered sugar, cocoa powder, milk, and vanilla extract in a large bowl until smooth. Spread in an even layer over the cooled bars, then cut into squares.
1/2 cup (114g) salted butter, 3 cups (320g) powdered sugar, 4 Tablespoons cocoa powder, 1/4 cup milk, 2 teaspoons vanilla extract