For the base: Lightly grease a 20cm square baking tin and line with baking paper. In a food processor, or using a rolling pin and bag, crush the biscuits. Pour into a bowl with the melted butter and mix to combine. Spoon into the baking tin and press down with the back of a spoon to flatten. Cover and put in the fridge to chill for 30 minutes.
For the filling: Combine the condensed milk, brown sugar, golden syrup and butter in a saucepan. Stir constantly over a low heat until the mixture has thickened, about 10 minutes. Pour over the chilled base, spreading evenly and put back in the fridge to chill completely. This should take about an hour.
For the topping: Melt both the milk and white chocolate. Smother the milk chocolate over the chilled filling and then randomly dollop half of the melted white chocolate over the top of the milk chocolate layer. Using a few drops of the pink food colouring, colour the remaining melted white chocolate and then dollop this on top of all the chocolate layers. Finally using a small palette knife lightly swirl the colours together to create your desired marbled effect.
Cover again and chill in the fridge until the chocolate has completely set. Then cut into slices and enjoy!