Swiss Meringue Buttercream

A beautiful silky smooth buttercream. It takes longer than the simple butter and icing sugar method, but isn't that difficult and is definitely worth it. Perfect for piping.

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   Put the egg whites, caster sugar and a pinch of salt into a glass or metal mixing bowl (the bowl for your stand mixer if you have one). With a balloon whisk (just a hand one – metal or plastic), lightly whisk together, then place the bowl over a pan of simmering water and whisk for a couple of minutes until the sugar has dissolved and the mixture is frothy. If you have a thermometer, keep whisking until the temperature reaches 71 degrees C. Without a thermometer, just check that the mixture is no longer grainy, by rubbing a little between your fingertips.
   Wearing oven gloves / with a tea towel because it is hot, transfer the bowl to your stand mixer, if you are using one. Fit the whisk attachment and beat on a medium speed for a minute or two until the steam dissipates, then switch the speed to the highest setting and whisk for 10 minutes. This can also be done with hand-held electric whisk fitted with the balloon whisk attachment, but your arm might ache a bit from holding it!
   After 10 minutes, you will have a glossy meringue. Detach the whisk and tap it against the side of the bowl a few times to remove any meringue that is stuck to it. You might also need to scrape it off with a spatula.
   Change to the paddle / K attachment (or the beaters on a handheld whisk). Chop the soft butter into rough cubes, then add about 1/3 to the meringue and beat on a low speed, then add in another 1/3, followed by the rest.
   As the butter mixes in, the meringue will lose some of its volume. If it becomes quite liquid, don’t worry – just keep mixing. Stop the mixer and scrape down the sides of the bowl to bring it all together again as and when required. After a couple of minutes, turn up the speed to medium. It may start to look curdled. Again, just keep mixing. If it doesn’t curdle, don’t panic either (it seems to depend on the temperature of the butter and the room itself).
   A few minutes later, even if it has curdled, the mixture will emulsify and become really thick, creamy with a silky texture, and glossy. At this point, add the vanilla extract and whisk for a further couple of minutes, again not worrying if it curdles when you add the liquid – it will sort itself out!
   It is now ready to spread or pipe onto cake(s).