Generously grease a 900ml/1½ pints pudding basin with butter, then chill in the fridge for 4-5 minutes.
When the basin is chilled, drizzle the golden syrup into the base of the basin.
In a large mixing bowl, whisk together the flour, baking powder, eggs, brown sugar and butter until the batter is smooth and well combined. Spoon the cake batter into the pudding basin.
Fold a large piece of aluminium foil in half and grease one side with butter. Wrap the pudding in the greased foil, greased side facing into the pudding. (Pleat the foil a few times as you wrap it to allow room for the pudding to expand.) Secure the foil to the rim of the pudding basin with string.
Place the wrapped pudding into a large steamer. (Alternatively, place the wrapped pudding into a large pan, pour in enough boiling water to reach halfway up the sides of the basin, then cover the pan with a lid.) Steam the pudding over a medium heat for 1 hour 30 minutes, or until cooked through, topping up the steamer with boiling water as necessary. (NB. The pudding is cooked through when a skewer inserted into the centre comes out clean.)
To serve, turn the steamed pudding out into the centre of a serving plate. Cut it into slices and drizzle over any remaining golden syrup, to taste. Serve with extra golden syrup and vanilla custard, if using.