Put the butter, sugar, cocoa powder and golden syrup in a saucepan on a low heat. Melt the butter, stirring, until everything is combined.
Tip in the sultanas and biscuits, then stir until everything is coated in the chocolatey mixture.
Line a 20cm cake tin with baking parchment. Tip in the mixture and press it down so that the top is even and flat.
Put the milk chocolate in one bowl and the white chocolate in another bowl. Melt both chocolates by microwaving for 30 seconds at a time, then stirring, then repeating until the chocolate is melted.
Pour the milk chocolate over the tiffin, then smooth with the back of a spoon. Pour the white chocolate on top, then use a skewer to draw swirls until a marble effect is created.
Put into the fridge to chill for 12 hours, then cut into 16 equal sized pieces.