Preheat the oven to 350F. Line a 9x13" baking pan with parchment paper, leaving enough of an overhang to easily luft out the finished bars. Set it aside.
In a large bowl, whisk together the flour, baking powder and salt until thoroughly combined. Set it aside.
Using either a stand mixer fitted with a paddle attachment or a hand mixer, beat the butter on med-high speed until creamy, 1-2 minutes. Then beat in the sugar for 1 minute, until fluffy. Beat in the egg, egg yolk and vanilla extract until combined. Scrape down the sides as needed.
On low speed, add the dry ingredients to the wet and mix until just combined. The dough will be thick and sticky.
Press the dough evenly into the prepared baking pan. Bake for 20-22 minutes, or until the edges are just starting to lightly brown. The center will still look very soft.
Place the baking pan on a wire rack and let it cool completely. Once cool, lift out with the parchment paper and frost.
For the frosting, in a medium bowl using a hand mixer or a stand mixer, beat the butter until creamy, 1-2 minutes. Then add the rest of the frosting ingredients and mix until combined. You can add more cream if you'd like a thinner frosting. Frost the cookie bars before slicing them. You can also decorate with sprinkles after frosting if you'd like to. Use a large, sharp knife to slice them.
Store these in an airtight container for 3 or so days. Don't stack them, as the frosting will not harden and set much.
