Chop the unsweetened chocolate into small pieces.

In a 2-quart saucepan, combine heavy cream, sugar, light corn syrup, butter, salt, and half of the chopped chocolate.

Heat the mixture on medium heat for about 5 minutes, stirring continuously with a wire whisk until bubbles start to form at the edge of the saucepan.

Reduce the heat to low and simmer for 10 minutes, whisking frequently to prevent scorching and ensure even cooking.

Remove the saucepan from the heat.

Whisk in the vanilla extract and the remaining chopped chocolate until the mixture is smooth and the chocolate is completely melted.

Let the sauce cool for about 5 minutes, whisking occasionally, until it thickens slightly.

Serve the hot fudge sauce warm over ice cream or other desserts.