Grate the rind of the orange. Put the butter and grated rind of the orange in a bowl and beat until soft and smooth. Sieve over the icing sugar and add the caster sugar, gradually beat the sugars into the butter mixture. Add the rum little by little, beating well between each addition, making sure not to curdle the mixture
Spread out the butter on a sheet of foil, cover and chill in the fridge until firm. Keep chilled until ready to serve.