Add the avocado oil to a large bowl. Add both white and brown sugars and whisk the mixture very well for at least a full minute.
Add the egg and whisk for another full minute.
Add the vanilla and flaky salt and whisk to combine.
Add the cocoa (sift if it's lumpy) and whisk to combine.
Switching from a whisk to a rubber spatula add in the flour, cornstarch, baking powder and baking soda. Stir to combine well.
Stir in chocolate chips if using.
Let the batter rest, covered in the bowl on the counter for 15 minutes.
Preheat oven to 350 degrees. Line a large cookie sheet with parchment paper.
Using a 2 inch scoop drop the batter onto the parchment in nice, round mounds. Leave 1 inch border along edges and good space in between so they don't stick together.
Sprinkle lightly with flaky sea salt. Bake for 10-12 minutes until they puff up and the tops are cracked all over.
Let cool for a couple minutes on the cookies sheet. Remove to completely cool on a wire rack.
Store at room temperature in an airtight container.