Melt the butter in a large heavy saucepan over low heat. Gradually add the flour and stir constantly until the mixture is chestnut brown. It may be necessary to turn off the heat completely at intervals so that the flour will not burn or become too dark.
When the desired color has been reached, add the tomato puree and stir well. Gradually add the hot stock and powdered beef bouillon, using a wire whisk to insure smoothness. Bring the mixture to a boil, then reduce the heat to the lowest degree possible. Simmer 2 hours or until the sauce has been reduced to about 1 1/2 quarts.
Add the bottled brown gravy sauce, salt, and caramel color if desired. Remove from the heat and strain through a fine sieve. Cool to room temperature and store refrigerated.