Whisk the flour, cornstarch, baking soda, and salt together in a medium bowl. Set aside.
In a large bowl using a hand-held mixer or stand mixer with paddle attachment, beat the coconut oil1 for 2 minutes on medium-high speed until smooth, about 1-2 minutes. Add the brown sugar and granulated sugar and beat on medium-high speed until light and fluffy, about 2 minutes. Add the eggs and egg yolks, and vanilla and beat medium-high speed until combined, about 1 minute. Scrape down the sides and the bottom of the bowl, as needed, and beat again for a few seconds to combine.
With the mixer running on low speed, slowly add the dry ingredients until just combined. To make this easier (and less messy), I use a 1/4 cup measuring scoop. Beat in the chocolate chips2 for 5-10 seconds. The dough will be thick. Cover and chill the dough for at least 30 minutes in the refrigerator (or up to 3 days). If chilling longer than 1 hour, allow the dough to sit at room temperature for 15 minutes before rolling as the dough will be hard.
Scoop balls of dough, about 1.5-2 Tablespoons per cookie, and place on the baking sheets about 3 inches apart. Bake for 10-11 minutes until slightly browned on the sides. The centers will look soft.
Remove from the oven and allow to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely. While the cookies are still warm, I like to press a few extra chocolate chips into the tops - just for looks!
The cookies stay fresh for up to 1 week when stored in an air-tight container. Baked cookies can be frozen for up to 3 months. Allow to thaw overnight in the refrigerator.1If you don't want any coconut flavor, use extra refined coconut oil. Using regular coconut oil you may have a slight coconut flavor depending on the brand.2To keep these dairy-free, use dairy-free chocolate chips. My favorite brand is Enjoy Life.Make ahead tip: You can make the cookie dough and chill it in the refrigerator for up to 3 days covered. Sit at room temperature for 15 minutes before continuing with step 4. You can also freeze unbaked cookie dough balls for up to 3 months. Bake frozen cookie dough balls for an extra minute.