In a heat proof measuring cup or small heat proof mixing bowl, heat the milk in the microwave for about 1-1 ½ minutes (be careful, you want the milk hot but not scorched) and set aside.
Melt the butter in a heavy bottomed sauce pan on medium heat.
Stir in the flour and cook, whisking constantly, until the paste cooks and bubbles a bit, about 2 minutes, but don't let brown.
Add the hot milk, whisking constantly as the sauce thickens.
Bring to a boil.
Add salt and pepper to taste, lower the heat and cook whisking constantly for an additional 2-3 minutes.
Remove from heat.
Chef's notes: See variations below. Add any additional spices at the last 2-3 minutes cooking time. Add a pinch of nutmeg to give it a nutty flavor. Add ½ cup cheddar cheese and a pinch of cayenne for a cheese sauce, adding the last 2 minutes cooking time. Add a slice of onion to the milk when heating, then remove, for an onion flavor. Add 2-3 Tablespoons of good Dijon mustard for a mustard sauce.