• Preheat oven to 350 degrees F. Line a 9" x 13" baking pan with heavy duty foil or parchment paper, letting the excess hang over the edges so it will be easier to lift the bars out. Lightly spray the foil-lined pan with non-stick spray. This will allow you to lift the bars out of the pan, for clean cuts.
• In a medium mixing bowl, whisk together flour, baking soda, and cornstarch; set aside.
• In a large mixing bowl with an electric hand mixer or in the bowl of a stand mixer beat the butter, brown sugar, and sugar until combined. Add in egg, egg yolk, and vanilla extract. Continue to whisk until well combined.
• Add in the eggs and vanilla extract and mix until combined.
• Add the dry ingredients to the wet ingredients and mix until completely combined and cookie dough has formed but don't over-mix. Add in the chocolate chips and mix again until evenly distributed throughout the dough.
• Transfer the cookie dough to prepared pan and press the dough evenly into pan using your hands or a spatula.
• Bake for 25-30 minutes or until the top is light golden brown and slightly firm. Let the cookie bars cool for 5 minutes and then lift bars out of the pan and transfer to a wire rack.

Make buttercream frosting

• While the cookie bars cool, make the buttercream frosting by creaming the butter together until smooth in a medium bowl. Add the powdered sugar, heavy creamy or milk, and vanilla extract. Beat for a few minutes until frosting is creamy, smooth and fluffy. Add in more powdered sugar to thicken the frosting or milk to thin it out. 
• Use a butter knife or offset spatula to frost the cooled cookie bars, top with sprinkles (optional), cut into 24 bars with a sharp knife. Enjoy!