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.