Add egg whites and caster sugar to a clean, heatproof bowl.
Place over a pan of simmering water and whisk constantly until the sugar has dissolved and the mixture reaches about 70°C (or until it feels smooth with no sugar grains when rubbed between your fingers).
Transfer to a mixer and whip on high speed until you get a thick, glossy meringue and the bowl feels completely cool.
Slowly add the softened butter, a little at a time, while mixing on medium speed.
Add vanilla and continue mixing until the buttercream becomes silky, smooth and creamy.
Keep whipping until you have that beautiful, velvety texture ready for decorating your cakes ✨
