Break the chocolate up into pieces and melt it in a double boiler. Once it has melted, add the butter in slivers along with the sugar. Let it cool down and then add the cream, egg yolks and Negrita rum. Pour the mix into a mould and let it stand for at least two hours in the fridge. Peel one orange with a potato peeler and cut the peel into thin strips. Crystallise it for 5 minutes in a saucepan with 100 g of sugar and 1/10 litre of boiling water. Carefully peel the rest of the oranges and cut them into thin slices. Serve the chocolate sprinkled with the crystallised orange peel and surrounded by the orange slices. Decorate with fresh mint leaves.