Whisk on medium speed until the mixture becomes meringue-like and has reached stiff peaks, about 3 mins. (You could use an electric whisk, but it will take considerably longer, around 35 mins.) Add the butter 1 tbsp at a time, whisking to combine before the next addition. It may look curdled and soupy but be patient and keep whisking until you have a smooth, fluffy buttercream. Whisk in the vanilla. If using this buttercream to ice a cake, change to the paddle attachment and set it on a low speed for a few minutes – this helps remove air bubbles from the icing and makes for a smoother finish to your cake. If you like, you can also add melted chocolate, freeze-dried fruit powder, malt extract or other flavourings.