Line a 11x7 inch baking tray with some greaseproof paper and set aside.
In a large bowl, melt the butter/margarine and Mars Bars over a bain marie or in short bursts in a microwave. Use the back of a spoon/spatula to break up the nougat, as this won't melt as well - don't worry about lumps though, it won't be perfectly smooth.
Add the rice crispies to the melted ingredients and mix well, ensuring all are coated.
Tip into your prepared baking tray and push down with the back of a spoon/spatula, ensuring an even finish right into the corners.
Melt the milk chocolate over a bain marie or in short bursts in the microwave before spreading over the top of your rice crispy base.
Place into the fridge to set for at least a few hours or ideally overnight. Cut into your desired portion sizes to serve.