Preheat your oven to 350°F (175°C) and prepare a 9x9 inch baking pan by lining it with parchment paper.
In a large mixing bowl, beat together the softened salted butter and sugar until light and fluffy, about 3-4 minutes.
Add in the large egg and vanilla extract to the butter-sugar mixture, beating until well combined and smooth.
Gradually sift in the all-purpose flour and baking powder, mixing until a thick dough forms.
Gently fold in the festive sprinkles with a spatula.
Spread the batter evenly into the prepared baking pan.
Place the pan in the oven and bake for about 20 minutes or until the edges are lightly golden.
Remove from the oven and let them cool in the pan for at least 10-15 minutes.
Prepare the frosting by blending powdered sugar, softened butter, cream cheese, vanilla extract, and milk until creamy.
Spread the frosting over the cooled bars.
Sprinkle additional festive sprinkles on top of the frosted bars.
Cut the cooled, frosted bars into squares and serve immediately.