Preheat oven to 350 degrees and line a 9" square pan (or your preferred pan) with parchment paper.

In a large bowl cream the butter and sugars together on medium speed until fluffy and light in color. Mix in egg and vanilla until combined.

In a separate bowl, combine flour, cornstarch, baking soda and salt. On low speed, slowly mix into the wet ingredients. Dough will be thick and look like it's not coming together but it will. Stir in chocolate chips.

Press dough into prepared pan and bake for 16-20 minutes or until the edges just start to turn brown. Let cookie cool completely in pan.

Frost with buttercream if desired.