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.
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.