If the vanilla is added during cooking, it will lose its flavour.
You can also use a digital thermometer equipped with a probe. Just drop the probe into the syrup.
For a firmer fudge, continue cooking until the thermometer reads 240°F (116°C).
The cooled syrup is very thick. Stirring it takes elbow grease! The syrup will begin to set suddenly. Do not stop stirring until you spread it in the pan, or it could seize in the bowl.
If desired, add toasted and chopped walnuts just before stirring. You can also substitute the vanilla for maple extract.