A really nice alternative to buttercream - not as sweet and sickly.
250g (10oz) sugar
5 large egg whites (200g)
500g (1lb) unsalted butter, softened
1. Slowly heat 250g sugar and water in a small saucepan without stirring. When it starts to boil turn up the heat and boil until the temperature reads 121C - once it reaches this temperature place the saucepan in a bowl of water for a few seconds to stop it getting any hotter.
2. Meanwhile, whip egg whites to stiff peak stage in a food mixer - use the balloon whisk attachment - whisk in the remaining 50g sugar.
3. With the mixer running, slowly pour the boiling sugar in to the egg whites and continue to beat. Take care not to pour the syrup over the sides of the bowl as it will set and not mix into the buttercream.
4. Beat the mixture for about ten minutes or until the mixture feels cool
5. Gradually whisk in the butter a little at a time - the meringue will collapse and look curdled, but just keep whisking until it forms a smooth, fluffy buttercream.