Sift together both flours, baking soda, baking powder, salt, and cornstarch into a bowl. Set aside.
Using a mixer fitted with a paddle attachment, mix the coconut oil and both sugars together until very light, about 5 minutes.
Add the eggs and egg yolk, one at a time, mixing well after each addition. Stir in the vanilla.
Reduce speed to low, add dry ingredients and mix until just combined, 5 to 10 seconds. Drop chocolate chips in and incorporate them by hand without breaking them. Press plastic wrap against dough and refrigerate for 24 to 36 hours. Dough may be used in batches and can be refrigerated for up to 72 hours.
When ready to bake, preheat the oven to 350°F. Line a large baking sheet with parchment paper or a nonstick baking mat. Set aside.
Scoop 1 ½ tablespoon sized scoops of dough onto the prepared baking sheet and flatten.
Bake until golden brown but still soft, 10 to 12 minutes. Transfer cookie sheet to a wire rack for 2 minutes, then slip cookies onto another rack to cool a bit more. Repeat the same process with the remaining dough.