This is a recipe from my mother's 1942 Good Housekeeping Cookbook

Hot Fudge Sauce

2 sq. 2 oz unsweetened chocolate                   1/2 cup water

1/8 tsp salt                                                     1 1/2 cups corn syrup

1 tsp vanilla

Cook chocolate and water over low heat 2 min or until melted and thick, stirring constantly.  remove from heat and slowly add the corn syrup and salt.  simmer gently 10 min. stirring occasionally.  add vanilla. serve hot or cold. 

Chocolate Sauce

2 sq. 2 oz unsweetened chocolate                    2 cups bottled milk or 1 cup evap milk and 1 cup water

1 1/2 tbs flour                                                  2/3 cup sugar                             1/8 tsp salt

2 tbs butter or salad oil                                     1 tsp vanilla

Add chocolate to milk in double boiler; heat until melted.  Beat whith egg beater 1 min until smooth. combine flour, sugar and salt in small bowl.  stir in a little of the chocolate mixture.  return to double boiler; cook until thickened, stirring.  Add buter and vanilla. 

I too did not know they made a canned sauce