Preheat oven to 350F (175C) and prepare a 13x9 pan by lining with parchment paper (or lightly grease and flour the pan). Set aside.
Combine butter and cream cheese in the bowl of a stand mixer (or use a large bowl and an electric mixer). Beat together on medium-speed until well creamed (about 1 minute).
Add sugar and gradually increase mixer speed to high, beating until well-combined and fluffy (about 1 minute).
Add eggs and egg yolk, one at a time, stirring well between each addition. Scrape down the sides and bottom of the bowl as needed to ensure ingredients are well-combined.
Stir in vanilla extract.
In a separate, medium-sized bowl, whisk together flour, baking powder, and salt.
Gradually add dry ingredients to wet, stirring on medium-low speed until completely combined.
Spread batter evenly into prepared pan and bake on 350F (175C) for 25-30 minutes or until center is set and edges are beginning to turn light golden brown.. Allow cookie bars to cool completely in pan before covering with frosting.
Frosting
Place butter in a large bowl and beat with an electric mixer until cream.
Gradually add sugar until completely combined.
Stir in vanilla extract and salt, stir until well-combined.
Spread over completely cooled sugar cookie bars. Decorate with sprinkles, cut into squares and serve.