Grease a 9 inch cake square cake tin or line it with baking paper
Set up a double boiler, or place a large heatproof bowl (metal or glass) over a pot of simmering water, making sure the pot doesn't touch the water.
Place the butter and golden syrup and chopped mars bars to the boiler/pot. Stir gently until melted and mix under fully incorporated. There may still be small lumps of nougat which is fine.
Remove the bowl from the heat and add the rice krispies. Mix thoroughly and transfer to the cake tin before the mixture cools. Press the rice krispies firm but gently into all corners of the tin and flatten on top.
Using a clean bowl with the double boiler, add the chocolate until melted
Pour the melted chocolate over the rice krispies, spread evenly and (if using) add green sprinkles over the top. Chill in the fridge until set.