Preheat oven to 375°F.
Butter or spray the bottom of a 9-inch springform pan, then place a parchment or waxed paper round inside to cover the bottom of the pan. Butter or spray the paper and the sides of the pan.
Sift together the flour, cornstarch, baking soda, and salt in a small bowl. Set aside.
Cream together the butter and sugars. Add the egg and egg yolk with the vanilla and beat to combine well.
Slowly add the dry ingredients to butter mixture and mix until just combined. Stir in the chocolate chips.
Transfer the cookie dough batter to the prepared pan and smooth surface evenly, leaving about 1/2-inch of space from the edge of dough to the sides of the pan for spreading.
Bake for 12 - 18 minutes, until browned and cooked through.
Cool for 10 minutes, loosen edges of cookie carefully with a plastic knife, then cool completely.
Remove springform ring and slide cookie onto a serving platter. Decorate with your favorite buttercream and sprinkles.