Prepare a pot of simmering water for a double boiler.

Add the egg whites and sugar into a bowl. Place the bowl of the double boiler and whisk the mixture for 3-5 minutes until the mixture is frothy and the sugar has completely dissolved. The best test if there are any traces of sugar left, is to rub a tiny amount of mixture between your fingers and you should not feel any sugar granules.

Pour the mixture into a bowl of a stand mixer fitted with the whisk attachment. Whisk on medium-high speed, until firm peaks form, about 10-15 minutes. It is important that firm peaks form or the buttercream won't achieve the right consistency. When the meringue has reached firm peaks and the base of the bowl is cool to the touch, change to the paddle attachment.

Add the cold cubes of butter one at a time, making sure each piece is fully incorporated before adding the next. After all the butter has been added, the buttercream will be thick, smooth and silky. Add in the salt and vanilla and mix for 30 seconds.

Transfer the meringue to a storage container.

For use the same or next day, store at room temperature.

For use 2-5 days later, refrigerate the mixture then bring the meringue to room temperature and beat with the paddle to make the mixture smooth and glossy again.

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