Cut baguette diagonally into 1cm-thick slices. Spread one side of each slice with softened butter. Arrange the slices of bread with the buttered side up in a 24cm lightly greased round heatproof Pyrex dish. Fill the dish, overlapping the slices of bread if necessary. Sprinkle some mixed peel over with half the combined lemon and orange zest.
Beat milk, sugar and eggs until well mixed. Add Cointreau and orange juice to combine. Pour half the liquid over the bread and allow the bread to soak up the liquid.
Arrange the second layer of buttered baguette slices over. Pour remaining liquid over. Press down the baguette slices slightly to allow the liquid to come up to the surface of the bread slices. Allow to soak for five minutes.
Stand dish in a roasting pan or baking tray. Pour in hot water to come halfway up the sides of the dish. Bake in a preheated oven at 180°C for 30 minutes. Remove dish from oven and sprinkle over with remaining lemon and orange zest. Continue to bake for a further 15 to 20 minutes. Insert a sharp knife into the centre of the pudding. If it comes out clean, the pudding is ready. Remove pudding from the oven and brush with golden syrup. Serve warm.