In a bowl, cream the butter and sugar together with an electric hand-held mixer until pale and fluffy.
Add half of the beaten eggs and continue to beat for 1–2 minutes. Stop, scrape the sides of the bowl, then add the remaining beaten egg and continue beating for 1–2 minutes. Scrape the sides again. Add the nutmeg, lime and lemon zest, and beat for a few seconds more.
With a metal spoon, gradually fold the sifted flour and baking powder into the mixture as lightly as you can, keeping to a dropping consistency by adding milk, if needed. Fold in the toffee or fudge pieces.
Bring a large pan of water to the boil and place a steamer with a lid on top.
Butter and lightly flour a 1.2 litre/2 pint pudding basin. Spoon the treacle (or golden syrup) into the bottom of the basin, then add the pudding mixture. Secure the lid and steam gently for three hours, adding more water to the pan if necessary – keep a close eye on the pan so it doesn’t boil dry. The pudding is cooked when a skewer inserted into it comes out clean.
Serve with custard and extra nutmeg grated over.