On medium-high heat, bring to a gentle boil, whisking constantly. Boil for four to five minutes until the mixture thickens (it may take longer depending on the quality of your saucepan, but so long as you are whisking, it won’t burn). Remove the pan from the heat. In a small bowl, whisk the egg yolks. Gradually whisk in about one cup of the hot-milk mixture. Pour the egg yolk mixture into the saucepan and, whisking constantly, simmer for two to three minutes until slightly thicker. Remove the pan from the heat.