Preheat the slow cooker if necessary; see the manufacturer's instructions. Butter 4 metal pudding moulds, each 250 ml (8 fl oz), and base-line each with a circle of nonstick baking paper, checking first that they will fit in the slow cooker pot.
Beat the butter and sugar in a bowl with a wooden spoon or hand-held electric whisk until soft and creamy. Gradually add alternate spoonfuls of egg and flour until both have all been added and the mixture is smooth. Mash the banana on a plate, then beat into the pudding mix. Stir in the dates, then divide between the moulds.
Cover each one with a square of foil and stand in the slow cooker pot. Pour boiling water into the pot to come halfway up the sides of the moulds. Cover with the lid and cook on high for 2–3 hours or until the tops of the puddings spring back when pressed with a fingertip.
Lift the moulds from the slow cooker pot using a tea towel and remove the foil. Loosen the edges with a knife and turn out on to plates. Pour the toffee sauce into a small saucepan, add the chocolate and warm through, stirring until the chocolate has just melted. Drizzle over the puddings and serve immediately.