3 cups whole milk
6 tbsp (3/4 stick) unsalted butter
4 1/2 tbsp all-purpose flour
1/2 teaspoon salt
Heat the milk in a small pan until hot but not scalded or boiling. While the milk is heating, melt the butter in a medium saucepan. When it is foamy, whisk in the flour until smooth. Stir-cook for 2 minutes over medium heat. Do not let the flour brown. It should be blonde in color.
Add several tablespoons of the hot milk and whisk vigorously. Continue to add milk in small increments, whisking until it is incorporated. Eventually, you will be able to add it in greater amounts until it has all been incorporated into the sauce.
Add the salt and cook over medium-low heat stirring often until the sauce thickens to the consistency of heavy cream, 3 to 5 minutes.
Remove the pan from the heat to prevent further thickening. Use immediately or cool to room temperature and whisk until smooth before using.