Melt the white chocolate in a heatproof bowl over a pan of simmering water (ensuring the bowl doesn't touch the water), or in the microwave in short bursts. Leave to cool.

Put the butter in the bowl of a stand mixer and beat on high speed for 3-4 mins until pale and creamy. Or, do this in a large bowl using an electric whisk. With the motor running on low speed, drizzle in the cooled melted white chocolate, then turn the speed up slightly and whip until combined. Add the icing sugar in three additions until you have a creamy buttercream, then whisk in the vanilla extract and a small pinch of salt. If you like, switch to the paddle attachment and beat on low speed for 3-4 mins to knock out any air bubbles – this results in a smooth buttercream that is good for piping.