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.