Weigh out your butter, sugar, golden syrup and condensed milk into a thick based pan. Heat this gently until the sugar has dissolved and the butter has melted.
Meanwhile, melt the 100g mint chocolate in your preferred way. Break it up into chunks and either melt it in a glass bowl over a pan of simmering water or do it in the microwave. I prefer to do this in the microwave. I put the chocolate on high for one minute, take it out, give it a stir and return it to the microwave for a further 10 seconds. Repeat again. Usually after 1 minute, 20 seconds is fine and it naturally melts the rest of the chocolate from the heat of the bowl. Add the chocolate to the condensed milk mixture.
Weigh out your cereal. I used a mixture of Rice Krispies and porridge oats but you can use any cereal you want so long as it adds up to 125g. Cheerios would work. Put all the cereal into a large mixing bowl and then pour in the chocolate mixture. Mix it so it is well coated.
Mix in the Malteser Mint Buttons ensuring they are evenly spread.
Put the mixture into your prepared tin. Use the back of a spoon or a potato masher to level it out.
Melt the remaining 200g of mint chocolate in the same way as before. Pour this melted chocolate on top of the crispie layer.
Put in the fridge to set.
When set, cut the crispie into the number of slices you want.