1. Melt Butter: Melt the butter in a heavy sauce pan over low heat, then add flour. Cook for three minutes, whisking steadily. Butter and flour will foam.
While you're cooking the butter and flour, heat the milk in your microwave for 1/2 minute at medium power. The milk should be hot but not too hot to touch.
2. Add Milk: Whisk in milk and cook, whisking constantly, until thickened. The sauce should cling to the back of a spoon and a finger drawn through it should leave a trail.
3. Add Egg Yolks: Whisk egg yolks with mustard, salt, and pepper and any other herbs or spices you’re using. Then remove pan from heat and whisk in yolks.
4. Add Cheese and Other Ingredients: Whisk in cheese (along with any leftover Parmesan) until completely melted. Add any other ingredients. Allow mixture to cool to room temperature - about 15 minutes.
