Preheat the slow cooker to high. Grease six 150ml ramekin dishes. In a mixing bowl beat together the sugar and butter until light and fluffy. Beat in the eggs, one at a time until the mixture is pale and creamy. Add the milk and the vanilla extract. Add the orange zest and Cointreau and mix well. Fold in the flour, cocoa powder and baking powder until the mixture is well blended.
Divide the mixture between the ramekins. Cover each one with a piece of pleated greaseproof paper and secure with tin foil and kitchen string.
Place the puddings in the preheated slow cooker over four stacked layers of greaseproof paper and pour in enough boiling water in to come half way up the ramekins. Cover and cook for 90 minutes.
Take the moulds out of the water bath and remove the paper and foil. Invert over a serving plate and gently tap out. Serve hot.