Prepare the Pan: Lightly grease a 9x13 inch baking pan. Line it with parchment paper, leaving an overhang on the long sides to use as handles for easy removal.

Start the Base: In a medium saucepan, combine 1 cup of unsalted butter, granulated sugar, light brown sugar, and 1/2 cup of milk. Place over medium heat.

Boil the Mixture: Stir constantly until the butter is melted and the sugars are dissolved. Bring the mixture to a full, rolling boil that cannot be stirred down.

Time the Boil: Once at a rolling boil, set a timer and continue to stir constantly for exactly 60 seconds. This step is crucial for the bars to set properly.

Add Final Base Ingredients: Immediately remove the pan from the heat. Quickly stir in 1 cup of creamy peanut butter, 1 teaspoon of vanilla extract, and the salt until completely smooth. Add the 3 cups of quick-cooking oats and stir until every oat is evenly coated.

Press into Pan: Scrape the oat mixture into the prepared pan. Use a greased spatula to press it into a firm, even, and compact layer. Set aside to cool while you prepare the frosting.

Make the Frosting: In a large bowl with a hand mixer or a stand mixer, beat the 1/2 cup of softened butter until light and creamy. Add 1 cup of creamy peanut butter and 1 teaspoon of vanilla, and beat until well combined and smooth.

Finish the Frosting: Gradually add the powdered sugar, mixing on low speed until incorporated. Add 2-4 tablespoons of milk, one at a time, beating until the frosting is light, creamy, and easily spreadable.

Assemble and Chill: Spread the finished frosting evenly over the cooled oat base. Cover the pan and refrigerate for at least 2-3 hours, or until the bars are completely firm.

Slice and Serve: Use the parchment handles to lift the entire slab out of the pan. Place on a cutting board, slice into 24 squares, and serve.