Line a 9x9-inch pan with parchment and lightly grease. Set aside. If using a stand mixer, fit it with the whisk attachment. Have all your ingredients ready to go.
Pour the cold water (1/2 cup) into your stand mixer's bowl or a large glass bowl. Sprinkle the gelatin in and allow to bloom.
Pour the remaining water (2/3 cup) into a saucepan, and pour in the monk fruit allulose sweetener. Bring to a boil over medium heat, allowing the sweetener to completely dissolve.If you have a kitchen thermometer, heat the mixture until it reaches about 210°F (100°C). Don’t worry if you don’t have one—simply bring it to a rolling boil and let it cook for about 2 minutes. Unlike regular sugar, sugar alcohols don’t caramelize or reach the same high temperatures used in traditional candy making, so boiling for 2 minutes ensures the mixture is hot enough for the recipe to work properly.
At this point, you’ll want to work fairly quickly so the mixture stays hot. Turn your stand mixer to low speed just long enough to break up the bloomed gelatin, then carefully pour the hot syrup into the bowl, aiming for the gelatin rather than the sides of the bowl to minimize cooling.Increase the mixer to high speed and whip continuously for about 15 minutes. Add the salt around the 8-minute mark, then the vanilla extract (and liquid stevia, if using) around 12 minutes. By the end of mixing, the marshmallow mixture should be thick, glossy, and fluffy, holding stiff peaks. If you’re using a glass mixing bowl, it should feel only slightly warm to the touch.
Turn off the mixer and immediately pour the marshmallow mixture into your prepared baking dish. Work quickly, as the mixture begins to set fast. Don’t worry about scraping every last bit from the whisk or bowl—trying to get it all can cause the marshmallow to firm up before you get it into the pan.
Allow the marshmallows to set uncovered at room temperature for at least 6 hours, or preferably overnight, until they are firm and easy to handle. Gently lift them out of the pan and cut into squares using a lightly greased knife for clean edges.Unlike traditional marshmallows, these keto marshmallows aren’t overly sticky, so there’s usually no need to coat them in powdered sweetener. If they do feel tacky, simply rub a tiny amount of coconut oil on your knife or hands to prevent sticking. In very warm or humid weather, you can lightly dust them with powdered allulose or your favorite powdered keto sweetener if desired.Store the marshmallows in an airtight container in a cool, dry place for up to 2 weeks. For longer storage, freeze them for up to 3 months.
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Notes
I greased my knife to slice into 4 columns and 5 rows making 20 total.