To make the buttercream, beat the butter for 5 minutes using a handheld mixer. We want the butter to go paler in colour. Then add the sifted icing sugar in two halves, beating well in between. Add the vanilla and just enough hot water so that the buttercream is spreadable. Fill a piping bag, that’s fitted with a nozzle (I use a wilton 1M), with the buttercream and pipe rosettes onto the cupcakes.