Prepare your 9x13 pan by sprinkling a thin layer of toasted coconut onto the bottom. Use about half of the coconut and reserve about half for later.
In a small saucepan, add water and sprinkle the gelatin over top.
Let the gelatin sit for 5 minutes to "bloom" (soften).
Meanwhile, whip your egg whites on high power until they have stiff peaks.
Add cream of tartar and mix briefly to combine.
Once the gelatin has bloomed, stir mixture until it is smooth. If some lumps remain, place the saucepan over low heat and melt the lumps.
Add the powdered sweetener to the gelatin mixture and stir to combine.
Transfer the mixture to a glass measuring cup with a spout. If some lumps of gelatin still remain, strain the mixture as you transfer it.
Turn the mixer onto low speed and slowly drizzle in your gelatin mixture.
Once the gelatin is incorporated, turn the mixer back up to high speed and whip the mixture for 3 or 4 minutes until it increases in volume and becomes thick and shiny. Add the vanilla in the last minute, after the gelatin has all been incorporated.
Quickly transfer the marshmallow into your prepared pan and spread it out.
Sprinkle some additional toasted coconut on top of the marshmallow.
Let the marshmallow set for at least an hour (overnight is good). Cut into squares.