Prepare the Dough: In a large mixing bowl, cream together the softened butter and granulated sugar using a hand mixer or stand mixer until the mixture is light and fluffy, about 2–3 minutes. Add in the egg and vanilla extract, beating until well combined. In a separate bowl, whisk together the flour, baking powder, and salt. Slowly add the dry mixture into the wet ingredients, mixing on low speed until a soft dough forms. The dough should be thick but smooth.

Bake the Cookie Bars: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment paper for easy removal. Press the dough evenly into the pan, smoothing it out with a spatula or the back of a spoon so the top is level. Bake for 23–25 minutes, or until the edges are lightly golden and the center is set. Be careful not to overbake—the bars should remain soft. Remove from the oven and allow them to cool completely in the pan before frosting.

Make the Frosting: While the bars are cooling, prepare the frosting. In a clean bowl, beat the softened butter until creamy. Gradually add the powdered sugar, one cup at a time, mixing well after each addition. Add in the vanilla extract and 2 tablespoons of cream or milk, beating until the frosting is smooth and spreadable. If you want a slightly thinner frosting, add an extra tablespoon of cream. For fun variations, mix in a drop or two of food coloring to create a festive look.

Frost the Bars: Once the cookie bars have cooled completely, spread the frosting evenly over the top using an offset spatula or the back of a spoon. Be generous with the frosting so each bite is sweet and creamy. Sprinkle with colorful sprinkles for a classic sugar cookie finish.

Slice and Serve: Using a sharp knife, cut the frosted cookie slab into 16 even bars. Wipe the knife between cuts for clean edges. Arrange on a serving plate and enjoy! These bars are soft, buttery, and perfectly sweet—just like a frosted sugar cookie but in an easy-to-share form.