Mix together in a bowl all the ingredients except the chocolate, butter and cream.
Place the chocolate, butter and cream into a saucepan over a low heat and allow the chocolate to melt, stirring to combine.
Pour the chocolate over the biscuit mix and stir everything well until combined.
Meanwhile line a 20cm/8in cake tin with a large sheet of foil pressing it carefully into the tin. Brush the foil with a scant coating of vegetable oil.
Spoon the chocolate mix into the foil lined cake tin and push down well. Cover the cake with more foil and place in the refrigerator. Chill for about two hours. Remove from the tin and lift off the foil. Cut into wedges and serve with vanilla ice cream.