(click on the text to highlight each stage):

1. Lightly grease a 22x 32 swiss roll tin (at least 1.5cm deep) with melted butter.
2. Melt the butter in a large heavy-based saucepan over a low heat. Add the evaporated milk, the 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, for 10 minutes, until the mixture reaches 116-118°C (the soft-ball stage). To test, take the pan off the heat and drop a small blob of the mixture into a bowl of cold water. It should be firm but still malleable.
3. Plunge the base of the pan into cold water for a few minutes to stop the mixture cooking further, then leave to cool to 110°C (about 5 minutes). Stir with a wooden spoon until the mixture thickens, loses it shin and begins to sound almost gritty on the pan base. Quickly pour into the prepared tin and leave to go cold, then cut into squares.