Combine the chocolate chips and the sweetened condensed milk in a small saucepan over medium heat.
Whisk or stir constantly until the chocolate has melted and the ingredients have combined smoothly, about 3-5 minutes. Add extract if desired and stir to combine. The sauce should be quite thick and will very slowly pour off of a spoon.
If a thinner sauce is desired, add 2 tablespoons of heavy cream and whisk or stir well to combine with the sauce. Add additional cream for an even thinner sauce. Serve warm. Transfer any remaining sauce to an airtight container and refrigerate.
Notes
COOK’S NOTE: The hot fudge will firm up as it cools. Reheat it in the microwave (at half power!) stirring frequently, until the sauce is smooth and stirrable once again.