In a medium saucepan set over medium heat, whisk the chocolate, sugar, heavy cream, corn syrup, cocoa powder, and salt together until the chocolate is melted. Whisking occasionally, allow the mixture to come to a boil. Boil for 3 minutes. Use an instant-read thermometer to check the temperature; the hot fudge is done when it reaches 220°F (104°C). (If you don’t have a thermometer, 3 minutes is usually the amount of time.)

Immediately remove from heat and stir in the butter and vanilla extract until combined.

You can use the sauce immediately for ice cream or other desserts. To store, let it cool completely and store it in an airtight container in the refrigerator for up to 1 week. Hot fudge will thicken as it cools and in the refrigerator. To reheat, warm it over low heat on the stove or in the microwave.