In a microwave-safe bowl (or saucepan on low heat, stirring constantly) melt the chocolate on half power, in 30-second increments, stirring in between, until melted and smooth.
Remove from microwave and stir in the sifted cocoa powder until smooth. The mixture will be thick. Set aside.
In a medium saucepan over medium heat, combine the sugar, corn syrup, cream, salt and water. Stir constantly and bring to a simmer.
Simmer for 4 minutes. Remove from heat and stir in vanilla and butter.
Add the melted chocolate mixture and stir until smooth.
Serve warm, over ice cream.
To store: refrigerate in an airtight container for up to 3 weeks. Reheat on low power in the microwave, stirring several times, until warm and smooth.
Storing Instructions: Refrigerate in an air-tight container for up to 3 weeks. Warm in lightly in the microwave to serve.Make ahead Instructions: Make hot fudge sauce up to 3 weeks ahead of time and store it in an airtight container in your fridge. Heat slowly in the microwave, stirring every 30 seconds.Freezing Instructions: Allow hot fudge to cool completely and store it in a freezer safe jar or bag for up to 3 months. Allow it to thaw overnight in the refrigerator. Reheat the fudge slowly in the microwave, stirring every 30 seconds and being careful not to let it get too hot or burn.Recipe adapted from Cook's Illustrated Cookbook.