In a large bowl, put agar powder, whey protein, and ⅓ cup water. Mix well, and let sit for 1 hour.
After the 1 hour, heat remaining ⅓ cup water to a boil. Turn off heat.
Begin to beat the agar whey mixture with an electric mixer using the whisk attachment (if available).
Slowly add in the hot water, and continue to beat with electric mixer.
Once it is thick and no longer hot, add the marshmallow extract and sweetener.
After a total of 10 minutes of beating (or 15 minutes if you use stevia), you'll begin to reach "stiff peak" stage. This is when you (turn off and) lift the beater from the mix, and you see peaks of fluff that are stiff and don't fold over. That means you're done beating!
Using a spoon or rubber spatula, gently place your fluff in a jar, and enjoy!
Store in an airtight container in the refrigerator.