1-Preheat your oven to 350°F (175°C). Prepare a 13×18 inch rimmed sheet pan by greasing with butter or spraying with nonstick spray, then line the bottom with parchment paper, leaving an overhang for easy removal.

2-Melt the butter in a microwave-safe bowl in short bursts until just melted and slightly cooled.

3-In a large mixing bowl, whisk together the light brown sugar and granulated sugar. Pour in the melted butter and whisk until smooth and combined.

4-Add eggs and vanilla extract; whisk until the mixture is uniform.

5-Stir in baking soda, baking powder (or cornstarch), and salt.

6-Gradually fold in the all-purpose flour using a silicone spatula until a smooth dough forms.

7-Fold in 1 1/2 cups of the chocolate chips evenly into the dough.

8-Spread the dough evenly and thinly over the prepared sheet pan, pushing it all the way to the edges. The dough will be thin but will rise while baking.

9-Sprinkle the reserved 1/2 cup chocolate chips evenly over the top of the dough.

10-Bake for 14-18 minutes, depending on oven variations, until edges are golden and the cookie looks set but still slightly soft or underbaked in the center. Avoid overbaking to keep the cookie cake chewy and moist.

11-Allow the cookie cake to cool completely in the pan before frosting or slicing. For best texture, let it rest for at least an hour.

12-Beat softened butter on medium-high speed until creamy and light in color (about 5-7 minutes).

13-Gradually add powdered sugar one cup at a time on low speed, scraping the bowl, until fully incorporated.

14-Add vanilla extract, salt, and milk; beat on low speed until smooth and pipeable. Adjust milk quantity if needed for desired consistency.

15-Pipe frosting along the edges or over the cookie cake. Quickly add sprinkles before the frosting sets, as it develops a crust fairly quickly.