Italian meringue buttercream
Step by step
Add the caster sugar and egg whites to a heat-proof bowl and whisk au bain marie for 5 minutes until the egg whites are warm and the sugar has melted. You can test this by rubbing 2 fingers together, you should no longer feel sugar grains.
Pour the sugar mixture into the bowl of a Stand Mixer fitted with the Wire Whisk and mix on speed 8 until the mixture has cooled off to room temperature and a merengue consistency has formed.
Attach the Paddle Attachment and put on speed 6. Add the butter gradually, waiting until each piece is mixed in before adding more. Add the vanilla extract and salt.