Set the canola oil in a large bowl, add the sugars and whisk the mixture very well, for a full minute.
Add the vanilla and sea salt and whisk to combine.
Add the egg and whisk vigorously for a full minute.
Add the cocoa (if it's lumpy, sift it in) and whisk to combine. Add the cornstarch, flour, baking powder and soda and stir to combine.
Optional: stir in chopped chocolate or dairy-free chocolate chips (no more than ¾ cup).
Let the dough rest on the counter, covered, for 15 minutes. Preheat the oven to 175°C. Line two cookie sheets with parchment paper.
Use a 1.5 tbsp scoop to portion the dough, leaving about a 1 inch border around each cookie so they won't stick together.
Optional: sprinkle with flaky sea salt before baking.
Bake for 10 minutes. The cookies are done when they have puffed up and cracked all over.
Once cool, remove from the pan. Store at room temperature in an airtight container.