Melt the Stork in a pan over a low heat, you can use a little of this to lightly grease a 22x32cm swiss roll tin.
Add the evaporated milk, Muscovado sugar and vanilla extract and stir with a whisk until the sugar has completely dissolved.
Turn the heat up and simmer vigorously whisking every now and then until the mixture reaches 116-118°C (soft ball stage). To test take the pan off the heat and drop a small blob into a bowl of cold water, it should be firm but still malleable.
Plunge the base of the pan into cold water for a few minutes to stop it cooking further, then leave to cool to 110°C (5 minutes).
Stir with a wooden spoon until the mixture starts to thicken, loses its shine and starts to sound almost gritty on the base of the pan. Quickly pour it into the prepared tin and leave to go cold, then cut into squares.