Line a 20cm x 20 cm (8" x 8") square tin with baking paper.
Crush the biscuits to fine crumbs - either in a food processor or with the end of a rolling pin - and place in a medium size mixing bowl.
Roughly chop the Maltesers, leaving some larger chunks, and put them in the bowl with the biscuit crumbs.
In a small sauce pan over a low heat, place the butter, cocoa, golden syrup, 50g milk chocolate and 50g dark chocolate and heat, stirring, until the butter and chocolate have melted.
Leave the chocolate mix to cool for a couple of minutes, then pour over the Maltesers and biscuit crumbs.
Mix well until all the biscuit crumbs are coated in the chocolate mixture.
Pour into the prepared tin and smooth out, pressing down with the back of a spoon.
Melt the remaining 50g milk chocolate and 75g dark chocolate, either in the microwave or in a heatproof bowl set over a pan of barely simmering water.
Once melted, pour over the chocolate Maltesers biscuit base, and spread out to cover the slice.
Sprinkle over the roughly chopped Maltesers and place in the fridge for 2-3 hours to set.
Take out of the fridge 30 minutes before you want to cut it. Using a sharp knife, carefully cut into 16 pieces.
Store in an container in the fridge.