Beat the sugar, butter, and black treacle together for 2 minutes or until light and fluffy. Add the eggs one at a time, and add a spoonful of flour with each one to prevent curdling.
Mix in the remaining tablespoon of golden syrup (20g), flour, baking powder, and salt until blended. Add enough milk to give the batter a “dropping” consistency; meaning it can fall off of a spoon when shaken.
Spoon the batter into the prepared basin and cover with the lid, parchment side down. Secure the cover by tightly tying it with kitchen twine under the lip of the basin, and make a string handle to make it easy to remove the pudding later.
Place the basin in the steamer, and fill it with boiling water to halfway up the side of the basin. Bring the water to a gentle simmer and steam for 2 hours, or until a skewer inserted through the foil and into the pudding comes out clean. Add more water as needed throughout the cooking process to keep the water level halfway up the basin.
Remove the pudding from the steamer and take off the cover, then let it stand for 5 minutes. Run a flat-bladed knife round the edge to loosen it, then invert it onto a serving plate.
Drizzle the pudding with a little extra golden syrup if desired, then serve immediately with pouring custard.