In a heavy 8 to 10-inch skillet, melt the butter over moderate heat. When the foam begins to subside, stir in the flour and mix thoroughly. Pour in the stock and, stirring constantly with a whisk, cook over high heat until the sauce thickens and comes to a boil.
Reduce the heat to low and simmer the sauce for about 3 minutes to remove any taste of raw flour. Then stir in the capers, vinegar, salt and a few grindings of pepper. Taste for seasoning and serve hot from a small bowl or a sauceboat.