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. Serve warm.
The sauce should be quite thick and will very slowly pour off of a spoon. Transfer any remaining sauce to an airtight container and refrigerate. Enjoy!
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.