Melt the Butter: In a large bowl, melt the salted butter in the microwave in 30-second intervals until fully melted.
Mix in Sugars: Add the granulated and brown sugars to the melted butter. Using a hand mixer, beat until the mixture is smooth and fluffy.
Add Eggs and Vanilla: Mix in the egg, egg yolk, and vanilla extract until well combined.
Combine Dry Ingredients: Stir in the cornstarch, baking soda, and salt. Scrape down the sides of the bowl, then add the flour and mix on low speed until just combined.
Fold in Chocolate Chips: Gently fold in the semi-sweet chocolate chips.
Prepare the Pan: Transfer the dough to a large round pan, pressing it out to about ½ inch from the edges.
Bake: Preheat the oven to 325°F (165°C) and bake for 18-20 minutes. For a softer cookie texture, slightly underbake. Allow to cool completely before frosting.
Make the Whipped Buttercream Frosting
Cream the Butters: In a large mixing bowl, beat the softened butter and shortening together until light and fluffy.
Add Flavor: Mix in the vanilla extract and heavy whipping cream until well combined.
Incorporate the Sugar: Gradually add the powdered sugar, mixing thoroughly after each addition to avoid a sugary mess.
Add Color: If using food coloring, mix it in now until you reach your desired shade. For multiple colors, divide the frosting into separate bowls.
Chill: Refrigerate the frosting for at least 20 minutes. If it’s too runny, chill longer or add more powdered sugar to thicken.