Put the butter, sugar, orange rind and eggs in a large bowl, then sift in the flour, cocoa powder and bicarbonate of soda and beat well until creamy. Stir in the chocolate.
Grease the inside of a 1.2 litre (2 pint) pudding basin and line the base with nonstick baking paper. Turn the mixture into the basin and level the surface. Cover with a double thickness of nonstick baking paper and a sheet of foil, securing them under the rim of the basin with string.
Bring a 5 cm (2 inch) depth of water to the boil in a large pan. Lower in the pudding basin and cover the pan with a lid. Steam for 1¾ hours, topping up the water occasionally, if necessary.
Heat the sugar, butter and orange juice gently in a small pan until the sugar dissolves. Bring to the boil and boil for 1 minute. Stir in the dates and cook for 1 minute. To serve, invert the pudding on to a serving plate and pour the sauce over the top. Serve with pouring cream or custard on the side.