Separate the egg whites from the egg yolks. I highly suggest doing this in a small bowl one by one, then adding them to your larger bowl. This will prevent you from accidentally getting any yolk mixed in.
In a medium/large glass bowl, mix together the sugar, cream of tartar, and egg whites.
Place your bowl on top of a pot of simmering water. An inch of water will do here. We are looking for a double-boiler kind of situation.
Using a small whisk or spatula, mix the egg whites slowly until it reaches around 155 degrees. I highly recommend a digital thermometer for this!
Once you're at temp, remove your bowl off the stove (be careful the bottom and sides will be hot) and add your vanilla extract.
Using an electric mixer, whisk your sugary egg whites for 3-5 minutes or until you have nice stiff peaks.
Frosting will be very sticky. Using a pastry bag is the easiest way to apply your frosting but not necessary.
If you'd like to toast your marshmallow meringue, decorate your cake or cupcakes as desired, and then using a mini torch toast the edges/sides/peaks until you are happy with your masterpiece.