Whisk together flour cornstarch baking soda and salt in a medium bowl and set aside
In a large bowl using a stand mixer, mix the butter and brown sugar beat until completely combined
Add stevia and beat well about 3 minutes
Add egg and vanilla beat on high speed about 2 minutes
Scrape down the sides and up the bottom of the bowl and beat again as needed to combine
You can add chocolate chips if you like in the last step
Cover the dough and chill at least 2 hours or up to 1 or 2 days
Then preheat the oven and make a ball about 1/5 tablespoon
Put it on the baking sheet, bake about 12-13 minutes
Until lightly browned on the sides. The center look very soft