Sprinkle the gelatin over half of the water in a deep mixing bowl (or bowl of a stand mixer) and let it bloom (absorb) for 5-10 minutes.
In a small saucepan, combine the remaining water, allulose, and salt.
Heat over medium until the allulose fully dissolves. For a firmer texture, optionally boil for 5 minutes to reduce excess water and slightly reduce the stickiness.
Using a stand mixer with a whisk attachment or a hand mixer, start mixing the bloomed gelatin on low speed.
Slowly pour in the hot syrup while mixing.
Gradually increase speed to high and whip for 10 minutes or until thick, glossy, and tripled in volume.
Add the vanilla extract towards the end of mixing.
Line a loaf pan with parchment paper and dust the bottom with arrowroot powder if you have it. Pour the marshmallow mixture in quickly and shake gently to smooth the top.
Let sit at room temperature for 6-8 hours until firm.
Dust with arrowroot powder to prevent sticking.
Cut into squares, dust further with arrowroot powder, and store in an airtight container.
Video
Notes
-Allulose is counted as a carbohydrate on labels, but since it is not absorbed like regular sugar, it contributes almost 0 net carbs.