If you’re using vanilla bean pods, use a knife to cut the pods lengthwise and scrape the beans into the bowl of a stand mixer. Add the egg whites, sugar, and salt.  

Place the bowl over a pan of gently simmering water. Whisk the mixture constantly until it reaches 65°C. 

Remove the bowl from the heat and place it on the stand mixer fitted with the whisk attachment. Whisk on medium for 8 to 10 minutes or until the bowl feels cool to the touch. It’s really important for the meringue to cool completely before adding the butter. 

Slowly add the butter a piece at a time. Initially, the mixture might look quite soupy, then as you add more butter, it might look curdled. This is completely normal. Once you’ve added all the butter, keep whisking until smooth. This could take a few minutes.  

Switch to the paddle attachment and mix it on a low speed for 1-2 minutes to get rid of any air bubbles. Taste the buttercream and add in more salt or vanilla if needed, until it reaches your desired flavor. Use immediately.