Line a swiss roll tin 9 x 13 inch (23 x 33cm) with parchment paper.
Melt the butter on a low heat in a wide heavy bottomed saucepan. Stir in the evaporated milk, water and sugar. Bring to a gentle simmer.
Cook on a very low heat, stirring constantly, until it reaches the soft ball stage, approx. 45 minutes. To test, drop a teaspoon of the liquid fudge into cold water, it is ready when a smooth soft ball forms.
Allow the fudge to cool in the pan for 3 minutes before whisking in vanilla. Immediately pour into the prepared tin and smooth the surface.
Cut while still warm.