Put the milk and cream in a saucepan and bring slowly to the boil. Meanwhile, whisk together the egg yolks and sugar until pale and very creamy. As soon as the milk and cream come to the boil, pour on to the egg mixture and whisk well. Return to the pan and place over a gentle heat, or in a basin over a pan of simmering water, stirring continuously until it has thickened. Serve hot.