1.  Put egg whites and sugar into the bowl of your stand mixer (make sure it’s heat proof) – whisk them like you mean it and combine the  sugar into the egg whites. 
2. Place the bowl onto a pot with an inch of simmering water and continue to stir them  every few minutes while heating for about 10-15 mins until you can carefully dunk your fingers into the bowl to feel for any sugar granules between your thumb and index finger. You don’t want a gritty buttercream so make sure all the sugar is dissolved. 
3. Transfer the bowl onto the mixer with the whisk attachment and whisk until the meringue is a very stiff peak. This takes a while so don’t rush it. sip some coffee and let it do its thing. 
4. Ok when you are certain your meringue is stiff enough, start adding your fats a little blob at a time, your meringue is going to fall and look soupy and curdled (PANIC) just kidding, this is normal, keep adding the fats until they are all in.
5. Add vanilla extract now if using and continue to whip until the curdled mess comes together into silky smooth buttercream.    

Extra Details:

 Ready to use the buttercream now? I recommend switching to a paddle just for a minute to knock out the excess air bubbles before you frost your cake. If your icing cupcakes or using this for something else than you can skip this.