Preheat oven to 350°F (177°C) and lightly grease a 12-inch or larger pizza pan.
Whisk together flour, cornstarch, baking soda, and salt in a medium bowl; set aside.
Using a mixer, cream softened butter with brown and granulated sugars on medium speed until smooth, about 3 minutes.
Add egg and vanilla extract; beat on high speed until fully combined, about 1 minute.
Add dry ingredients and mix on low speed until just combined.
With mixer on low, fold in chocolate chips until evenly distributed; dough will be thick and sticky.
Transfer dough to the pizza pan and flatten with fingers to about 3/4 inch thick, keeping dough centered and not reaching edges.
Bake for 20–24 minutes until puffed and lightly browned. Avoid overbaking.
Cool cookie pizza on pan set on cooling rack before slicing or garnishing.
Optional: press a few extra chocolate chips into the warm top for decoration.
Store leftovers covered at room temperature for 2–3 days or refrigerate up to 5 days.