Preheat the oven to 350 degrees F. Spray a 9x13 baking pan with cooking spray or cover with parchment paper and set aside.
Make the bars
In a large bowl beat together butter, cream cheese and sugar using a stand mixer or hand-held mixer on medium speed for 2 minutes until blended, light and fluffy.
Add vanilla, egg and egg yolk and beat for 3 minutes, stopping to scrap down the edges to get it well mixed. Set aside.
In a smaller bowl whisk together flour, baking powder, baking soda, cornstarch, and salt.
Add half the flour mixture to the wet ingredients and mix on low until just combined, add the rest of the flour making sure to scrap down the edges until fully incorporated, stopping when you no longer see flour.
Transfer dough to prepared baking pan and press it down with a spatula or with your hands, covering the whole pan evenly. The dough will be very sticky and thick so if you need to grease your fingers to help keep it from sticking.
Bake for 20- 25 minutes or until bars are lightly brown around the edges and a toothpick comes out clean.
Cool completely on a wire rack for about 30-45 minutes.
Make the buttercream frosting
Whisk butter with a metal attachment on medium-high speed until light and fluffy, almost completely white about 5 minutes.
Add powdered sugar, vanilla and heavy cream and mix on low speed until combined then turn up the speed to high and whisk until very fluffy and light, about 2 minutes.
Add two drops of food gel if desired and mix until you no longer see white, scraping the edges down if needed.
Once your bars are cool, frost them and add sprinkles, if desired.
Remove bars from pan and cut them into squares.
Be sure to use room temperature butter. If you forget to plan ahead, you can microwave butter for 10 second intervals until soft. Don’t overdo it and use melted butter, or your bars will come out flat and greasy.Don’t skip the cream cheese, it helps make the bars soft and fluffy.I prefer to use gel food coloring, because it doesn't affect the texture of the frosting. As always, use what you have and adjust.Additions/SubstitutionsI use unsalted butter, but you can use salted if that is what you have. Just omit the 1/2 tsp of salt from the bars.You can use almond extract in place of vanilla extract if you prefer.Adding raspberry extract to the frosting (or any other flavor) will let you customize your bars for any occasion.How to store sugar cookie barsStore in an airtight container at room temperature for up to 2 days or in the refrigerator for 5 days.Baking Sugar Cookie BarsUse a metal baking dish if you have one. Metal performs better for making this recipe (even heat distribution). A glass dish will work in a pinch.Be careful not to overcook your bars! Overcooking, they will be too dry. Undercooking, the center will deflate, and it will be too doughy.Mine are perfect at 23 minutes. However, all ovens are different! Set your timer for 20 minutes and check every few minutes until your toothpick comes out clean.They should be golden around the edges. The bars will continue to cook and set as they cool.About the buttercreamIf you prefer to, you can use half and half or milk. Heavy cream is always preferred as it makes the frosting nice and creamy!If the buttercream is too sweet for your tastes, you can balance it by adding a pinch of salt.