In to the bowl of a stand mixer, place the aquafaba, cream of tartar, and ¼ cup (50 g) sugar. Using a wire whisk attachment, whip at medium low speed for two minutes. You can also use an electric hand mixer.
Increase the speed to medium high and whip for four minutes. The mixture will turn white and increase in volume.
Continue to whip for another two minutes while lightly sprinkling the remaining sugar, one tablespoon at a time, over the top of the meringue.
Increase the speed to one click away from high, add the optional vanilla, and whip for one minute more.
Lift the wire whisk or electric beater out.
With a flexible rubber spatula, turn the meringue onto the top of the pie, and lightly and evenly spread it around in swirls. Make sure the meringue reaches all the way to the edge of the crust and that there are no gaps. Use the backside of a spoon to make some peaks and valleys.
Pre-heat a broiler to low. Place the pie on a rack so that the top of the pie is about 4 inches away from the broiler and brown for just 20 seconds. If you don’t have a window and light in your oven, keep the door open and watch like a hawk, as the peaks can burn in 30 seconds.