Line a 23cm square tin with non stick baking parchment.
Place all of the ingredients (except vanilla extract if using) into a large saucepan and place over a gently heat to melt together.
When everything has melted, bring the mixture up to the boil then turn down the heat to maintain a fast simmer. Stir the mixture every few minutes.
When the mixture reaches 115°c - 118°c take the saucepan off the heat and leave to stand for 5 minutes. If you don't have a thermometer, use the cold water test as described above to test the fudge.
Add the vanilla extract at this point if using. Now, using a large wooden spoon, beat the mixture until it cools and thickens. This can take a little while but you will see it change.
Once it becomes thick and difficult to beat, pour the mixture into the ready lined tin and leave to set.
When cold, remove from the tin and cut into small squares.