Grease and line a 20 x 20cm/8" x 8" baking tin with parchment paper. Put the mars bars and butter in a heat proof bowl and place over a pan of barely simmering water. Mixing with a whisk, melt until the mixture is smooth. Pour over the rice krispies in a mixing bowl and mix until all the ingredients are evenly combined. Press evenly into the prepare baking tin and set aside. Melt the milk chocolate in the microwave or over a pan of barely simmering water. Spread the melted chocolate over the rice krispie mixture and leave to set in a cool place. Once set slice into squares and serve!