Melt the butter in a heavy-bottomed saucepan.
Stir in the flour and cook, stirring constantly, until the paste cooks and bubbles a bit, but don't let it brown, about 2 minutes.
Add the hot milk, continuing to stir as the sauce thickens. Bring it to a boil. Add salt and pepper to taste, lower the heat, and cook, stirring for 2 to 3 minutes more. Remove from the heat.
To cool this sauce for later use, cover it with wax paper or pour a film of milk over it to prevent a skin from forming.
Add enough liquid to mix to make a smooth paste. Add remaining water, bring to a gentle boil and cook 1 to 2 minutes.
Yields: 2 cups