Line a 8" square tin with greaseproof paper. I grease the tin first with a little butter to help the paper stick to it
In one pan, put the condensed milk and dark chocolate and melt together on a low heat, stirring together as it melts
In another pan at the same time, put the condensed milk and white chocolate and melt together on a low heat, stirring together as it melts
Once both mixtures are fully melted and combined, take the two pans off the heat and stir the icing sugar into each mixture
Put alternate blobs of each mixture into the tin until they are used up. Use a cocktail stick to swirl the mixture together a little bit
Put in the fridge overnight, or for 3-4 hours to set
Remove from the tin and slice into pieces to serve. Store in the fridge for 2-3 weeks