While the bars cool completely in the pan on a wire rack (this is crucial—frosting a warm base is a melt-y mess!), I make the frosting. I beat the softened butter until it’s smooth, then gradually add the sifted powdered sugar, milk, vanilla, and salt. I start with 2 tablespoons of milk and add more as needed to reach a thick, but spreadable consistency. Once the bars are completely cool, I spread the frosting over the top with an offset spatula in big, swoopy swirls. Then, I immediately shower it with a blizzard of colorful holiday sprinkles so they stick. The hardest part is waiting for the frosting to set a bit before cutting into neat squares.
Pro Tips for Best Results
The key to the perfect soft-yet-firm bar is in the creaming step. I tested this three different ways: creaming for 1 minute, 3 minutes, and 5 minutes. Three minutes was the sweet spot. One minute left the bars a bit flat and dense, while five minutes incorporated almost too much air, making them cakey. Three minutes of beating the butter and sugar until truly light and fluffy creates the ideal tender structure.
Letting the bars cool completely before frosting cannot be overstated. I was once too eager and frosted them while they were still slightly warm. The frosting melted into a glazy, runny pool and soaked into the cookie, making the top layer soggy. It was a sad sight. Now, I wait at least an hour, sometimes two, until the pan feels completely cool to the touch. Patience here guarantees a beautiful, distinct layer of frosting.
For the cleanest cuts and prettiest presentation, I use the parchment paper sling to lift the entire frosted slab out of the pan and onto a large cutting board. I use a large, sharp chef’s knife and wipe it clean with a damp cloth between each cut. This method gives you perfectly sharp edges and prevents the knife from dragging and marring the frosting. It makes them look professionally made.
Common Mistakes to Avoid
My first time making these, I used cold butter straight from the fridge. It wouldn’t cream properly with the sugar, no matter how long I beat it, leading to a greasy, dense dough that baked up unevenly. Your butter should be softened to room temperature—you should be able to press your thumb into it easily and leave a clear indentation. This is the foundation of your entire bar, so don’t rush it.(See the next page below to continue…)