Melt butter in a heavy pan over low heat.

Gradually stir in the flour, mixing until completely smooth. (A silicone spatula is the best tool for this job.)

Heat the mixture, over low heat, stirring, for 10 - 15 minutes, until the flour is completely cooked and very slightly browned. Let cool.

Pour into an ice cube tray, making 14-16 cubes. Freeze.

To use, stir into simmering liquid, until it thickens as desired. 1 cube for a thin sauce, 2 for a thicker one, 3 for a very thick one, used as a base for other foods.