Whisk egg yolks together in a small bowl or measuring cup. Slowly whisk in about 1/2 cup of the hot milk and sugar mixture into the egg yolks. Reduce the heat of the milk and sugar to in between a low and medium-low heat. While whisking the milk and sugar mixture, pour the egg yolk mixture into the milk and sugar. Continue to cook the mixture over a low heat until slightly thickened, about 4-5 minutes, stirring with a wooden spoon constantly. To test if it's thick enough, dip the spoon in the mixture and bring it out. Run your finger down the back of the spoon, and if the mixture stays seperated, it thick enough. Remove from the heat.