Lightly grease two 8x8 square baking pans OR one 9x13 baking pan with pan spray and line the pans with a strip of parchment. I recommend spraying the pans once more to grease the parchment.
Add 1/2 cup water to the bowl of a stand mixer fitted with the whisk attachment. Sprinkle the gelatin over the water and immediately stir to moisten any dry gelatin if necessary. Set the gelatin mixture aside to bloom while you make the allulose syrup.
In a medium heavy-bottomed saucepan, cook the allulose and the remaining 3/4 cup water, undisturbed, to 240 degrees F on a candy thermometer (soft ball stage). Pro tip: Keep a pastry brush in a cup of water next to the stove and periodically use it to wash down any crystallization that forms on the sides of the pan while the syrup cooks.
Once the allulose syrup reaches 240 degrees F, carefully add the hot syrup to the gelatin mixture in the bowl of the stand mixer. Start the mixer on low and run it while you add the vanilla and salt. Turn the mixer speed up to medium and whip until the marshmallow begins to thicken and lighten in color. At this point, you can turn the mixer up to high speed without the liquid making a giant mess. Whip on high for at least 10 minutes. You will know it's done when the marshmallow is VERY thick, glossy, cool to the touch, and holds firm peaks.
Working quickly, use a rubber spatula to scrape the marshmallow into the prepared pans. Use an offset spatula to smooth the surface as much as possible (you can oil both spatulas to help prevent sticking if you want to). Allow the marshmallows to set for at least four hours or overnight.
Using a blender, grind the additional allulose for dusting until it's the consistency of confectioners sugar. I use 1/4 cup of allulose and grind it in my Nutribullet.
Use the parchment paper to lift the marshmallow out of the pans. Using an oiled knife, trim the edges and then slice the marshmallows into strips. Dust the surfaces of each marshmallow strip with the powdered allulose. Then slice the marshmallow strips into squares and dust the cut sides. I find that I don't need to oil the knife when I cut the strips into squares.
Store the marshmallows in an airtight container. Or, lightly cover the container with a paper towel and allow them to dry out overnight, undisturbed.