In the top of a double-boiler over hot water, melt the chocolate squares. Stir in the sugar. Gradually stir in hot coffee, mixing thoroughly. Add the milk and continue cooking until all particles of the chocolate are dissolved and the mixture is smooth, about 10 minutes. Pour into a jar. Cover and chill. When ready to serve, stir in the chilled Coca-Cola. Serve over ice cubes in tall glasses.
For a beverage, top with whipped cream. For a dessert, add a scoop of vanilla ice cream. Makes 5 cups.