Crush biscuits in food processor or in a plastic bag with a rolling pin. Melt 100g butter and allow to cool slightly.
Mix melted butter, crushed biscuits, coconut and condensed milk. Mix in M & Ms. Place in the fridge to set while you make the topping.
Melt cooking chocolate and extra butter in a bowl set over hot water or in a double boiler if you have one. When melted, spread over the slice. Refrigerate until set.