Lightly grease four small pudding basins with a little butter. Put the remaining butter, sugar, orange zest, eggs, flour, cocoa powder and baking powder into a large bowl and beat until smooth and thoroughly combined. Add enough milk to give a dropping consistency.
Spoon the mixture into the pudding basin and smooth the surface. Cover with greased greaseproof paper and foil, both pleated in the middle. Secure with string.
Put the basins into a steamer or saucepan of simmering water, making sure the water comes halfway up the side of the basin. Cover and steam, topping up with boiling water as needed, for about 30 minutes.
While the puddings are cooking, make the chocolate sauce. Melt the chocolate in a bowl over simmering water and add the golden syrup. Add enough hot water to mix to a coating consistency.
To serve, turn the puddings out onto warmed plates, garnish with orange segments and top with a spoonful of chocolate sauce.