Grease a 1litre/1¾ pints pudding basin with butter. Put the golden syrup in to the bottom of the basin.
In a separate bowl, using an electric hand whisk, mix the flour, baking powder, butter, eggs, sugar and treacle until well combined.
Pour the mixture into the basin and cover with a piece of greaseproof paper.
Take a piece of tin foil and make a pleat down the middle, then place this over the top of pudding basin and tie it in place like a lid. The pleat allows the foil to expand during cooking.
Place an upturned saucer or a ramekin in a deep saucepan and sit the pudding on top. Add boiling water until it comes half-way up the basin. Place a lid on the pan and simmer gently to stream the pudding for about 2¼ hours, topping up the water as necessary.
To check if the pudding is cooked, insert a metal skewer into the pudding. If it comes out clean it is ready.
Carefully remove the pudding from the pan. Loosen the edges by running a table knife around the pudding and turn out onto a plate.
To cook in the microwave, spoon the mixture into the pudding basin until three-quarters full, then smooth the surface with the back of a spoon. Loosely cover with clingfilm (allow room for the clingfilm to inflate as the mixture rises) and place in the microwave for five minutes. The pudding is done when a skewer inserted into the centre comes out clean. Leave to stand for five minutes, before turning out onto a deep plate and remove the basin.