Grease a 9-inch square pan and set aside. You can line the pan with aluminum foil first if you’d like to be able to lift the whole block of fudge out easily.

In a large saucepan over medium heat, combine the cream, sugar, butter, corn syrup, chocolate, and salt.

Cook, stirring, until the sugar has dissolved, the chocolate has melted, and the mixture is smooth.

Continue to cook the mixture, stirring occasionally to keep it from sticking, until the fudge has reached the soft ball stage, about 5 minutes after it starts boiling.

If you’re using a candy thermometer, this is between 234-240°F (112.2-115.6°C). If you don’t have a candy thermometer, you can drop a teaspoon of the candy into very cold water - it will form a soft ball which flattens on removal from the water when it has reached this stage.

Remove the pan from the heat, stir in the vanilla, and let it cool, without stirring, until it is 110-115°F (43-46°C), about 10-15 minutes.

Once it has cooled, beat the fudge on low speed with an electric mixer, until it just starts to thicken (about 2 minutes).

Quickly spread the fudge in the prepared pan, smoothing the top.

Let it cool completely so it sets before removing it from the pan and cutting it into squares.

Store the fudge in an airtight container at room temperature or in the fridge for up to two weeks, or freeze it for up to 3 months.