Add all the ingredients to a pot except the chocolate malted balls and the vanilla extract, and stir over medium heat until the sugar has dissolved.

Bring the mixture to a boil, and stir continuously for 15–20 minutes until it has reached 118 ºC on a sugar thermometer.

Take it off the heat and beat it vigorously for a further 10 or so minutes until it starts to ‘fudge’ on the sides of the pot and becomes very stiff.

As it starts to stiffen, add the vanilla extract and the malted balls, stir briefly and immediately tip the fudge into the prepared tin, pressing it into the corners and smoothing it out. Leave it to cool and set before removing from the tin and cutting it into squares.