Add the egg whites, sugar and salt to a large heatproof bowl and place over a pan of simmering water. Gently whisk just until the mixture is hot to the touch and you can no longer feel any grains of sugar. Remove the bowl from the heat and transfer to the bowl of a stand mixer. Whisk the meringue for 3–4 minutes until it begins to hold stiff peaks but is still very warm. Stop the mixer and lift the whisk from the bowl.

Use a blowtorch to caramelise the surface of the meringue, making it nice and toasty, as you would with a marshmallow over a fire. Make sure to blowtorch the meringue attached to the whisk as well. Whisk the meringue for a further 30 seconds before repeating the process. Do this blowtorching at least 7–8 times until the meringue has taken on a warm caramel tone. Whisk the meringue for a few minutes more until it is no longer hot. Turn the mixer to medium speed and slowly incorporate the butter, a few pieces at a time. Once all the butter has been added, add the vanilla and continue mixing until a buttercream texture is formed. Switch to the paddle attachment, turn the speed to low and mix until the buttercream is super silky and devoid of any air bubbles.

Spread the buttercream over the top and sides of the cake, decorating as you please.

Kept covered the cake will keep for up to three days.

Tip: If the buttercream is too loose after adding the butter it will be because the meringue was too hot and needed to be whisked a little longer. If this happens, place the bowl in the refrigerator for 15 minutes, then re-whisk until thick and creamy. If the buttercream separates after the butter is added, the butter was likely too cold, but if you continue whisking it will eventually become smooth and creamy.