Seperate the egg whites making sure not to get any yolk in the whites. Eggs will separate better cold, but allow egg whites to come to room temperature before you make the meringue.
Fill a medium pot with about an inch or two of water and bring to a simmer (not a boil).
Use the metal bowl of you stand mixer and make sure it is spotlessly clean and free of any grease. In the bowl, whisk together the egg whites, sugar and cream of tartar.
Place the bowl over the pot of simmering water making sure the bottom of the bowl is not touching the water. Essentially creating a double boiler.
Continue whisking until frothy and egg whites are warm to the touch. If you feel the mixture between two fingers it should be smooth and not grainy.
Transfer the bowl to your stand mixer fitted with the whisk attachment. Beat until stiff and glossy peaks form. This could take about 10 minutes.
Add the vanilla in during the last 30 seconds.
Using a piping bag and large star tip, pipe rosettes on your chilled frosted cake. You can also spread it on with an offset spatula. Make sure you chill your cake first. We recommend chilling the cake while you make the meringue. A chilled cake will help the meringue hold form and not soften your frosting too much when you char the meringue.
Lightly char the meringue with a kitchen torch.
Notes
If you want to fully cover the cake in meringue you will need to triple the batch.