Preheat oven to 350f degrees. Line a 9"x 9" baking pan with parchment paper.
Beat together 1 stick (1/2 cup) salted butter, softened well and 1 cup sugar until fluffy, then beat in 1 large egg and 1 teaspoon vanilla. Finally, beat in 2 cups all-purpose flour and 1 teaspoon baking powder, a little at a time, until you have a thick dough. Gently stir in 1/4 cup festive sprinkles.
Spread batter into prepared baking pan. This will take a few minutes because it is thick. Bake at 350F degrees for 20 minutes or until golden brown.
Once it's finished baking, take cookie bar out of the oven and allow to cool fully. Once it is cooled, carefully lift the sides of the parchment paper to pull the whole cookie bar out of the baking pan.
To make the frosting, blend together 2 cups powdered sugar, 1 stick (1/2 cup) salted butter, softened, 3 Tablespoons cream cheese, softened, 1 teaspoon vanilla extract and 2 Tablespoons milk. Spread frosting generously on cookie bar, and top with additional sprinkles.
Cut into squares and serve immediately or let frosting set for a firmer feel.
Video
Notes
Do not overmeasure your flour or your dough will end up crumbly. As you can see there isn't a lot of liquid in this mixture. If you pack your measuring cup with flour, you will end up with way too much. If you do not have a kitchen scale then the best method to use is the spoon and scoop. Spoon the flour into your measuring cup (do not pack it down). Once it's full, take the flat end of a butter knife and wipe off the excess.
If you start mixing and you think you have dough that is too crumbly, add a Tablespoon of milk a little at a time until it looks like my pictures and video above.