Preheat oven to 350°F (177°C). Spray a 9-inch pie dish or 9-inch round cake pan with nonstick spray. Set aside.

In a large bowl using a handheld mixer or stand mixer fitted with paddle attachment, beat the butter and brown sugar together on medium speed until creamy, about 3 minutes. Beat in egg, egg yolk, and vanilla on medium-high speed until combined. Scrape down the sides and bottom of the bowl as needed.

In a separate bowl, whisk together flour, cornstarch, baking soda, and salt. Add the dry ingredients into the wet ingredients and beat on low speed until combined. The cookie dough will be quite thick. Add the chocolate chips and mix for about 5 seconds or until evenly dispersed.

Press the cookie dough evenly into the prepared pan.

Bake for 22–26 minutes or until the cookie cake is lightly golden brown all over the surface (could be up to 30 minutes, depending on your oven). Use a toothpick to test for doneness; if it comes out mostly clean with just a few moist crumbs, it’s done. You may want to cover the cake loosely with aluminum foil after 15 minutes to avoid heavy browning around the edges.

Remove from the oven and set the pan on a cooling rack to cool completely. While the cookie cake is still warm, you can press a few more chocolate chips into the surface of the cake. This is optional and only for looks!

Once cooled, use a sharp knife or metal spatula to loosen the sides of the cookie cake from the pan and transfer to a serving dish. (Or serve directly from the pan.)

Make the chocolate buttercream, if using.

Decorate the cooled cookie cake with frosting. Depending how much frosting you use, you may have some leftover. I used a Wilton 1M tip.

Cookie cake remains fresh covered tightly at room temperature for up to 3 days.