While the giant cookie cools completely in the pan on a rack (this is crucial—frosting a warm cookie is a melty disaster), I make the frosting. I beat the softened butter until smooth, then gradually add the sifted powdered sugar, milk, vanilla, and salt. I start the mixer on low to avoid a sugar cloud, then increase to high and beat for 2-3 minutes until it’s light and fluffy. I add a drop or two of gel food coloring and beat until uniform. Once the cookie base is completely cool, I spread the frosting evenly over the top with my offset spatula, and immediately shower it with a blizzard of festive sprinkles before the frosting sets.
Pro Tips for Best Results
The temperature of your butter is everything. “Softened” means it should hold a slight indent when pressed but still be cool—not greasy or melted. I leave mine out for about 45-60 minutes before starting. I’ve tested cold butter (won’t cream properly) and melted butter (makes greasy, dense bars), and perfect room-temperature butter is the key to the ideal texture.
Don’t overbake the bars. They continue to cook from residual heat in the pan. You want them just set. If you bake them until they’re golden brown all over, they’ll be dry. The moment the edges are lightly golden and the center springs back to a light touch, they’re done. I set a timer for 22 minutes as my perfect sweet spot.
For the sprinkles to stick best, add them immediately after frosting. If you wait even 5 minutes, the frosting will begin to form a slight crust and the sprinkles won’t adhere as well. I pour sprinkles into my hand and gently press them in over the whole surface to ensure good coverage and avoid a sad, sprinkle-bare spot in the middle.
Common Mistakes to Avoid
My first-time mistake was not lining the pan. I thought I could just grease it well. The bars stuck terribly, and I had to serve them as a “crumb cake” instead of neat squares. The parchment paper sling isn’t just helpful; it’s essential for getting the whole slab out in one beautiful piece for perfect cutting and presentation.(See the next page below to continue…)