Chocolate key lime pie

Name: Chocolate key lime pie
Cuisine: American
Ingredients: 300 g 11oz digestive biscuits (shredded wheat biscuits in Australia or ginger nuts for something different); 1 tbsp cocoa powder; 50 g 2oz soft unsalted butter; 50 g 2oz dark chocolate chips; 1 can condensed milk COLD; 4 pcs limes; 300 ml 1 cup double cream; 1 square of very good quality dark chocolate