In a small saucepan, bring milk and sugar to the boil and take off heat. While whisking the yolks constantly, slowly drizzle the hot milk in and continue whisking until the mixture thickens and coats the back of a spoon. Important: You need to add the hot milk in small amounts at first; otherwise the egg yolks will cook too quickly and the mixture will curdle. Once you’ve added half of the milk this way, the temperature of the egg yolks will rise enough to allow the rest of the milk to be added at once.