Grease a 1 litre pudding basin with butter and pour the golden syrup into the bottom.
Whisk the flour, baking powder, butter, eggs, sugar, and treacle in a bowl until well combined.
Pour the batter into the basin and cover with greaseproof paper.
Secure pleated tin foil over the basin as a lid to allow for expansion.
Place the basin on a saucer in a saucepan filled halfway with boiling water.
Cover and steam for about 135 minutes, topping up water as needed.
Check if cooked by inserting a skewer, then loosen with a knife and turn out onto a plate.