Grease a 9×9 inch (23x23 cm) pan very well and lay a large piece of parchment paper down (the parchment paper should be way bigger than the pan so after the fudge hardens you can use it to pull it out.)
Combine the granulated sugar, dairy-free milk, and margarine or dairy-free butter in a medium-sized saucepan over medium high-heat. Stirring constantly, bring it to a boil. Let it boil for 4 minutes still stirring, until it reaches 234ºF/112ºC or softball stage on a candy thermometer, then remove it from the heat.
Add the dark chocolate and marshmallows, stir or use a hand mixer until completely smooth. Add 1 tsp vanilla and stir again until incorporated, then pour into the prepared pan.
Let the chocolate fudge sit out for about an hour to harden, after it cools to room temperature you can wrap it and store it in the fridge for up to 1-2 weeks.