Preheat the oven to 350ºF (177ºC). Prepare a 13x18 inch sheet pan by spraying the sides with baking spray and lining the bottom with parchment paper.

Melt the butter: In a microwave safe bowl, heat the butter in 20-30 second intervals, stirring after each one, until the butter is melted. Set aside.

In a large bowl, add the brown sugar and granulated sugar. Pour the melted butter on top and whisk together until uniform. Add the eggs and vanilla, then continue whisking until uniform. Whisk in the baking soda, cornstarch, and salt.

Add the flour and use a silicone spatula to fold it in until smooth cookie dough forms. Reserve ½ Cup of the chocolate chips and set them aside, then add the remaining 1 ½ Cups of chocolate chips into the cookie dough and fold until evenly dispersed.

Add the chocolate chip cookie dough into the prepared sheet pan and use an offset spatula to spread it all the way to the edges in an even layer.

Sprinkle the reserved ½ cup of chocolate chips on top of the cookie dough and bake for 14-16 minutes, until it looks golden brown on the edges and set.

Let the cookie cake cool completely to room temperature before frosting.

With a hand mixer or paddle attachment on your stand mixer, cream the butter on medium-high until it’s creamy and light (almost white) in color. About 7 minutes.

With the mixer on low, add the sifted powdered sugar one cup at a time, scraping down after each addition and making sure each cup is fully incorporated before adding the next one.

Add vanilla, milk, and salt and mix on low for another minute until fully incorporated.

To create the look of the frosted cookie cake, begin by tinting the buttercream, if desired. Pipe stars all around the (cooled) cookie cake border and sprinkle with rainbow sprinkles.