1. Heat the waffle iron and have an electric mixer handy. In a bowl, whisk together the flour, sugar, cornstarch, salt, soda and cream of tartar (or baking powder). Use the whisk to combine the egg yolk, coconut milk, butter, and extract of choice.
2. Add the liquid ingredients to the dry ingredients and use a rubber spatula to gently stir. Switch to a folding motion toward the end to incorporate all the flour. If the resulting batter looks lumpy, that's okay. Expect the batter to be thick. Avoid over-stirring, lest the batter becomes over worked and yields a chewy, tough waffle. Set aside momentarily.
3. Use the electric mixer to beat the egg white for about 1 minute, or until it holds a 1 ½-inch peak. (See photo.) It will look solid white. Use the rubber spatula to gently fold in the egg white.
4. Spread a decent amount of batter onto the waffle iron, stopping short of the far outer edges since the batter will spread once the top is lowered. (I often don't fill up all the holes and let gravity distribute the waffle.) Cook the waffle until golden brown, 3 to 5 minutes. Darker waffles will be crisper (and stay crisper) than lighter ones. Aim for medium-brown, not light tan.
5. To remove the waffle, I use a pair of bamboo chopsticks to pry and lift the waffle from the iron. Place the waffle (which will be slightly soft) onto a cooling rack, where it will crisp up. Break up the waffle and enjoy warm. They'll soften as they sit but may be reheated to a crisp in the toaster oven.
Note: These waffles may be cooked beforehand, store in a plastic zip-top bag, and reheated in at 350F toaster oven until warm and crisp. They freeze well too.
Credit to http://vietworldkitchen.typepad.com/blog/2007/08/vietnamese-coco.html for the recipe