Christmas Sugar Cookie Bars

Name: Christmas Sugar Cookie Bars
Prep time: 20 min
Cook time: 20 min
Total time: 40 min
Cuisine: American
Ingredients: 1 stick (1/2 cup) salted butter, softened well ((not melted)); 1 cup sugar; 1 large egg; 1 teaspoon vanilla; 2 cups all-purpose flour ((see my notes below)); 1 teaspoon baking powder; 1/4 cup festive sprinkles; 2 cups powdered sugar; 1 stick (1/2 cup) salted butter, softened; 3 Tablespoons cream cheese, softened; 1 teaspoon vanilla extract; 2 Tablespoons milk; additional sprinkles