Preheat your oven to 350°F and grease a 9×13 metal pan, lining it with parchment paper.
In a stand mixer, blend together softened butter and granulated sugar until well combined. Add vanilla, egg, and egg yolk, mixing until incorporated.
With the mixer on low, add in flour, salt, baking soda, cream of tartar, and cornstarch. Mix until just combined.
Scoop the batter into the prepared pan and use a spatula or wet hands to evenly press the dough. Bake for 20-22 minutes until the edges are golden brown. Allow it to cool completely before frosting.
*For the frosting:*
In a stand mixer, beat together softened butter and powdered sugar until smooth (about 1 minute).
Add vanilla, salt, and 2 tablespoons of heavy cream or milk. Beat until smooth. Adjust the consistency with additional heavy cream/milk.
If you're coloring the frosting, add 1-2 drops of food dye and mix until incorporated.
*Assembly:*
Once the cookie bars have cooled completely:
Spread the frosting evenly over the bars.
Top with sprinkles and indulge!
Notes
These bars might rise and then sink a bit as they cool. For even bars, use the parchment paper to lift them from the baking dish. Use a serrated knife to level the edges, then frost and sprinkle away!