Put the butter and chocolate in a small pan and heat gently until the butter and chocolate have melted. Leave to cool.
Crush the biscuits to make fine crumbs. The easiest way to do this is to put them in a polythene bag and then crush them with a rolling pin.
Place the biscuit crumbs, the melted butter and the remaining ingredients in a bowl, and mix thoroughly.
Spread the mixture into an oiled tin or flat dish (such as an 8 x 12 inch, 20 x 30 cm, baking tin). Press down with a fork.
When the mixture is completely cool, cover with cling film and refrigerate for at least two hours.
When the cake has set firm, cut it into squares, which should then be returned to the fridge.
Key Ingredients: Chocolate, butter, walnuts, raisins