To Make Bars: Preheat oven to 350°F. Line an 8 or 9-inch square baking pan with parchment paper, leaving a little overhang on the sides.
In a medium bowl, stir together flour, salt, baking powder, baking soda, and cornstarch. Set aside.
In a large bowl using a hand-mixer, or stand mixer fitted with a paddle attachment, cream the butter until smooth, about 1 minute. Add sugar and beat on medium speed until fluffy and light in color. Beat in egg, egg yolk, and vanilla. Scrape down sides of bowl as needed.
Gradually add the dry ingredients to the wet ingredients in 3 parts, beating on medium speed until combined. Dough will be thick. Add sprinkles and fold in using a rubber spatula or mixing on low speed.
Press cookie dough evenly into prepared pan. Bake for 25 minutes until lightly brown on top, toothpick should come out clean. Allow the bars to cool to room temperature on a wire rack. Center will sink slightly, that’s okay.
Make the Frosting. In a large bowl, beat together cream cheese and butter until smooth. Add powdered sugar, vanilla, and salt. Mix until smooth.
Spread frosting evenly over cooled bars. Top with sprinkles. Slice, serve and enjoy!