Measure everything before beginning and reserve 20 minutes of uninterrupted working time for the hot-syrup and whipping stages. The setup should let you move between the saucepan, mixer, and pan without stopping to find ingredients.

Fit a stand mixer with its whisk attachment, or ready a handheld electric mixer, then line a 9x9-inch pan with foil and coat the foil thoroughly with coconut oil. An evenly greased lining helps the cured slab release without tearing.

Scatter the gelatin over the 1/2 cup cold water in the mixer bowl or a large glass bowl, combine with a fork, and leave it to hydrate for 10 minutes while preparing the syrup. The granules should absorb the water and form a swollen, gel-like mixture.

Heat the 2/3 cup water and your chosen xylitol or allulose in a saucepan over medium heat, adding the sweetener in the center and gently shaking the pan rather than stirring. When the sweetener has dissolved, maintain a rolling boil for about 2 minutes; an optional thermometer reading is approximately 210°F/100°C.

Run the mixer at low speed to loosen the hydrated gelatin, then promptly add the hot syrup into the gelatin mixture while avoiding the bowl walls and moving whisk. The heat should dissolve the gelatin, leaving a loose mixture ready to aerate.

Raise the mixer to high and whip continuously for about 15 minutes, adding the salt around minute 8 and the vanilla around minute 12. The foam should become thick and glossy, hold its shape, and leave a glass bowl feeling only mildly warm; the xylitol version will have less volume than the allulose version.

Stop mixing and immediately transfer the fluff into the prepared pan, spreading it without pausing to scrape every trace from the whisk. Xylitol fluff firms especially quickly, so prioritize getting the main batch into the pan.

Leave the pan uncovered at room temperature for 6 hours or preferably overnight, then lift out the slab and slice it with a greased knife. The marshmallow should feel set and offer gentle resistance when pressed rather than spreading like wet foam.