Silky Swiss Meringue Buttercream

Combine the egg whites and caster sugar in a clean, heatproof bowl. Set the bowl over a saucepan of barely simmering water to create a bain-marie, ensuring the bottom of the bowl does not touch the water.

Whisk the mixture constantly but gently until the sugar has completely dissolved. You can test this by rubbing a small amount between your fingers; it should feel perfectly smooth without any sugar grains. If using a thermometer, it should reach 160°F (71°C).

Transfer the mixture to the bowl of a stand mixer fitted with a whisk attachment. Whisk on medium-high speed until the meringue is thick, glossy, and has reached stiff peaks. Continue whisking until the outside of the bowl feels cool to the touch (about 10–15 minutes).

Reduce the mixer to medium speed and begin adding the softened butter one tablespoon at a time, ensuring each piece is fully incorporated before adding the next.

Don't panic if the mixture looks curdled or soupy during the butter addition; this is normal. Continue whisking patiently until the frosting suddenly transforms into a smooth, fluffy, and creamy consistency. Whisk in the vanilla extract.