Prep. We will need to work quickly while the marshmallows are hot, so prepare an 8x8 casserole (note 2) dish with parchment paper and grab your electric (or stand-up) mixer. Set aside.
Combine. Add 1/2 cup cold water to a large bowl. Sprinkle gelatin (3 tablespoons) on top and whisk until it dissolves. Set aside for 10 minutes.
Heat. Meanwhile, add 2/3 cup water, sweetener (2 cups), and a pinch of salt to a saucepan over medium-hgih heat. Whisk as you bring this combo to a boil, and continue to boil for 2 minutes.
Whisk. After 10 minutes, run the stand up or electric mixer in your bowl with the gelatin to break it into small clumps. With the mixer on, pour in the hot erythritol water, plus the vanilla extract (1 1/2 teaspoons). Make sure to transfer the erythritol water straight from a boil to the bowl (no resting time) -- we want to keep this combo as hot as possible, as once it starts to cool, it will set, which will lead to clumps in the marshmallows. Whisk (nonstop) on high for around 12 minutes. If using a hand mixer, move the mixer around continuously, you don't want the marshmallows to "set", and moving will prevent this. After 12 minutes, your marshmallows should be fluffy and will have doubled in size.
Shape. Pour marshmallow fluff into your prepared baking dish. Allow them to dry (uncovered) on the counter for 6 hours (note 3). Once firm, cut your marshmallows using a greased sharp knife (I rub melted butter along the edge of the knife with a paper towel). Store cut marshmallows in the fridge, and enjoy!