Lightly grease and fully line a 20cm (8in) square cake tin.
Place 300ml (½ pt) double cream, 100ml (3½floz) milk, 100g (3½oz) butter, 300g (10½oz) caster sugar and 150g (5oz) light muscovado sugar into a large heavy-based pan. Place over a low heat. Stir constantly until the butter has melted and the sugars have dissolved.
Increase the heat under the pan slightly to allow it to gradually come to the boil whilst stirring all of the time. Simmer the mixture, whilst continuing to stir, until it reaches 116°C ( 241°F) (soft ball stage).
Once the required temperature has been reached, remove the pan from the heat and leave it undisturbed to cool down to 110°C ( 230°F). This will only take a couple of minutes.
Add the vanilla and beat the mixture with the wooden spoon vigorously until the fudge has thickened and has lost its shine.
Pour the mixture into the prepared tin. Smooth it into the corners. Set aside to cool at room temperature for two or three hours.
Once fully cold and firm remove the fudge from the tin. Use a sharp knife to slice the fudge into bite sized pieces.