Remove from the heat. Take one cup of milk and empty, lightly and constantly stirring, over the egg yolks. Return the milk to the heat, constantly moving. Lower the temperature to medium and keep stirring until the mixture thickens. You'll know it's ready when the mixture sticks to the spoon and a canal is formed when you run your finger through it. It's very important to prevent the mixture from boiling; it should thicken over moderate heat. This takes about 10-12 minutes.