1. Preheat oven to 350°F (176°C). Line a cookie sheet with parchment paper.
2. Mix coconut oil and sugars together until well combined.
3. Add egg, egg yolk and vanilla extract and mix until well combined.
4. Add flour, baking soda, baking powder, cornstarch and salt and mix until well combined.
5. Add the oats and mix until everything comes together and is well combined.
6. Stir in the chocolate chips.
7. Drop rounded tablespoon sized balls of cookie dough onto the cookie sheet, about 2 inches apart. Press a few extra chocolate chips into the top of the cookie dough balls, if desired.
8. Bake 7-9 minutes. The edges should be lightly brown and the center may look slightly undercooked.
9. Let cookies cool for about 4-5 minutes on the cookie sheet, then remove to cooling rack to finish cooling.