While the base cools, make the frosting. In a clean mixer bowl, beat the softened butter on medium speed until it’s very creamy, about 2 minutes. Sift in the powdered sugar and cocoa powder to avoid lumps. Add the vanilla and pinch of salt. Start mixing on low speed until the dry ingredients are incorporated, then increase the speed to medium. Slowly drizzle in the heavy cream, and beat for 2-3 minutes until the frosting is incredibly light, fluffy, and spreadable. Once the base is completely cool, spread the frosting evenly over the top with an offset spatula. Cut into squares right in the pan.
Pro Tips for Best Results
The pan size is critical. A 9×13 pan will give you bars that are too thick and doughy in the middle, while the 10×15 inch jelly roll pan gives you the perfect lunch-lady thickness—a substantial but not overwhelming bar with a great frosting-to-base ratio. If you only have a 9×13, you’ll need to increase the bake time by 5-7 minutes and watch it carefully.
For the chewiest texture, let the bars cool completely in the pan before cutting, and then, if you can stand it, cover them and let them sit overnight. The oats continue to absorb moisture, and the flavors meld into an even more perfect, nostalgic bite. I tested eating them warm, cooled, and the next day, and the 24-hour bar was the absolute winner.
When making the frosting, if it seems too thick, add more cream a teaspoon at a time. If it seems too thin, add more sifted powdered sugar. The ideal consistency is like a thick, fluffy cloud that holds its shape when you spread it. For a truly authentic look, I sometimes drag a fork through the frosting in wavy lines to create that classic cafeteria texture.
Common Mistakes to Avoid
The first mistake I made was using natural peanut butter. The oils separated, the base was greasy, and it didn’t hold together. You need the stabilized, emulsified peanut butter for the right texture. This isn’t the time for health food—this is the time for pure, unapologetic nostalgia.(See the next page below to continue…)