In a medium bowl, combine flour, cornstarch, salt, baking powder, and baking soda.

In a large bowl using an electric mixer, cream butter and sugar together until light and fluffy.

Turn the mixer to high, add eggs one at a time. Mix until fully combined, scraping the bottom and sides of the bowl occasionally. Add vanilla extract and mix until combined.

Turn the mixer to low, add in the flour mixture in 3 portions. Mix until just combined each time.

Stir in chocolate chips and oats.

Scoop dough into 1.5 tablespoon portions. Cover or place in airtight container/bag and chill for 2 hours (or overnight). Preheat oven to 350 F. Drop dough portions on a lined baking sheet. Bake for 10-12 minutes.