In a saucepan over low heat, mix together the sugar substitute, cocoa, salt, and water. Heat until boiling; continue to boil and stir for 2 minutes. Mix in the 3 cups of low-fat or soy milk. When combined, add coffee and vanilla extract; heat through but do not boil.
Just before serving, pump with coffee press or whip the 3/4 cup of milk. Divide evenly into each of the mugs.