On the stove or on the griddle over medium/high heat (375 to 400°F [190 to 200°C]), place the butter in a medium saucepan.
Once the butter is melted, stir in the flour to create a roux.
As soon as the flour is thoroughly mixed in, begin pouring in the milk, a little at a time. As the roux and milk thicken, add more milk. Repeat until all the milk has