1-First Step: Prepare the pan and oven Start by preheating your oven to 350 degrees Fahrenheit. Line a 9×9-inch baking pan with parchment paper, leaving a little overhang on the sides if you can. This makes it much easier to lift the bars out after baking. Before you begin mixing, check that your butter is softened well but not melted. Real salted butter works best here because it gives the dough flavor and a tender bite. If you are baking in a warm kitchen, keep the butter out just long enough to soften, then get started.

2-Second Step: Cream the butter and sugar In a large bowl, beat together the softened butter and sugar until the mixture looks fluffy and light. This step helps add air to the dough, which gives the bars a softer texture after baking. Take your time here and mix until the sugar starts to blend in smoothly. Next, add the egg and vanilla extract. Continue mixing until everything is well combined. The dough will start to look smoother and richer at this point. If you are using a hand mixer, scrape down the sides of the bowl as needed so nothing gets left behind.

3-Third Step: Add the dry ingredients In a separate bowl, measure the flour and baking powder carefully. Spoon the flour into the measuring cup and level it off with a knife so you do not pack in too much. This is important because too much flour can make the bars dry or crumbly. If your baking powder is old, it may not give the bars the right lift, so check the date before you begin. Gradually beat the flour mixture into the wet ingredients until a thick dough forms. The dough may look dense, but that is what you want. If it feels crumbly, add milk 1 tablespoon at a time until it comes together. Be careful not to add too much, or the bars may turn soft in the wrong way.

4-Fourth Step: Fold in the sprinkles and spread the dough Gently stir in the 1/4 cup festive sprinkles. Mix just enough to distribute them through the dough without breaking them up too much. This keeps the colors bright and gives you those cheerful holiday spots throughout the bars. Spread the dough evenly into the prepared pan. A pastry roller can help press it into an even layer if the dough is sticky or a little stiff. Aim for a level surface so the bars bake evenly. Uneven dough can lead to overbaked edges and a softer center, so this step is worth the extra minute.

5-Fifth Step: Bake until lightly golden Bake the bars for about 20 minutes, or until they look golden brown around the edges. The center should look set, not wet, but the bars will still be soft. Avoid overbaking, since these bars are meant to stay tender and chewy. Once baked, remove the pan from the oven and let the bars cool completely. This part matters because warm bars will melt the frosting. Use the parchment paper to lift the cooled bars out of the pan before frosting.

6-Sixth Step: Make the frosting To make the frosting, blend the powdered sugar, softened butter, softened cream cheese, vanilla extract, and milk until smooth. The cream cheese gives the frosting a slight tang that helps balance the sweetness of the bars. If the frosting seems too thick, add a small splash more milk. If it seems too soft, add a bit more powdered sugar. A smooth frosting is easier to spread and looks more polished on top of the bars. Room temperature cream cheese is important here because cold cream cheese can leave lumps behind. If that happens, keep mixing until the frosting smooths out.

7-Seventh Step: Frost, decorate, and serve Spread the frosting generously over the cooled cookie bars. Then add additional sprinkles on top for a bright holiday finish. The frosting layer can be thick or thin depending on your taste, but a generous layer gives the bars that classic festive look. Cut the bars into squares and serve right away, or let the frosting set for a firmer texture. If you are serving them later, chilling them briefly can help the frosting hold its shape when sliced.