Half fill a medium pan with boiling water and simmer over a medium heat.

Place the butter, chocolate and golden syrup in a large heatproof bowl over the pan. Melt, stirring until silky smooth. Take off the heat and set aside to cool slightly.

In the meantime, put the biscuits into a zip bag and bash with a rolling pin until you have a mixture of crumbs and larger chunks. 

Fold the biscuit pieces and crumbs into the melted chocolate mixture, then the marshmallows. 

Once it is all nicely coated in chocolate, tip into a lined cake tray (24x24cm or 20x25cm). Smooth with a spatula and scatter toppings over the top, pressing slightly so they stick. 

Pop in the fridge for at least two hours (or overnight). Once firm, remove from the tin and cut into around 20 squares and enjoy.