Add eggs, muscovado and lemon zest if using. Stir well, then set aside for 15 mins to soak.
Stir the melted butter into the pudding mixture, tip into the tin, then scatter with demerara. Bake for 1½ hrs until firm and golden, covering with foil if it starts to brown too much. Turn out of the tin and strip off the paper. Cut into squares and serve warm.