Over a pot of simmering water place your mixer bowl (or a heatproof bowl) over the pot.

Add in the egg whites and sugar and stir until the sugar has dissolved. Continue to stir, this will take around 5 minutes. Putting the eggs over heat will technically cook them so you don't have to fear eating raw eggs.

Once the sugar has totally dissolved remove from the heat.

With whisk attachment begin to whip until the meringue on medium/high speed. Whip until the meringue is thick, glossy, and the bottom of the bowl feels neutral to the touch (this can take up to 5 minutes or so).

With mixer on medium speed, add vanilla, salt and soft butter cubes. Add the butter one piece at a time, until incorporated, and mix until it has reached a silky smooth texture (if curdles, keep mixing and it will come back to smooth).

*If mixture is too runny, refrigerate for about 1 hour until it comes together. continuing to beat until well combined.

Add additional flavours, purees, as desired.