Make a bain-marie/double-boiler by fitting a glass, ceramic, or metal bowl over a 6-inch saucepan and adding some water (2inches/5cm). Do not let the bottom of the bowl touch the water and make sure that the fit is loose enough that the steam can escape.
Place the saucepan onto a stovetop and bring the water to simmer.
Add the gelatin, granulated sweetener, and heavy cream to the bowl, stir well, and place it over the saucepan. Wait for the gelatin and sweetener to completely dissolve (about 5 minutes) in the heavy cream, but don’t let the mixture simmer.
When done - using a kitchen towel (the bowl will be very hot) - remove the bowl from the saucepan and set aside to cool a little bit.
Meanwhile, place the egg whites into a medium mixing bowl and, using a hand mixer or an immersion blender with a whisk attachment, beat them to stiff peaks. Then, add the slightly warm gelatin mixture and mix until fully incorporated.
Pour the mixture into a form (around 17x12cm/6x4-inches) lined with parchment paper and place into a fridge to chill for 1 hour.
Remove the form from the fridge and take the set marshmallow out onto the cutting board. Then, cut into 12 even pieces.
Finely chop the chocolate, if needed, and place it into a small ceramic or microwave-safe bowl. Slowly melt the chocolate by microwaving it for 15 sec. at a time (5-7 times) on 700W, stirring in between. It’s okay if there are a few lumps left after that, just keep stirring until they liquefy.
Dip each marshmallow into the chocolate and place onto a tray or cutting board lined with parchment paper.
Place the tray/cutting board with the marshmallows into a fridge for about one hour. Serve after the chocolate coating hardens.