4.I spread the batter evenly into the prepared pan and bake it for 20 to 25 minutes, or until it turns golden brown.
5. Meanwhile, for the frosting, I heat 1/2 cup of butter in a large saucepan over medium heat until it starts boiling. I let the butter turn a delicate brown and then remove it from the heat immediately.
6. I add the powdered sugar, vanilla extract, and milk to the browned butter. I whisk it all together until smooth; the consistency should be thicker than a glaze but thinner than frosting.
7. Using a spatula, I spread the brown butter frosting over the warm bars. It’s easier to spread the frosting while the bars are still warm.