Grease a 1.2-litre pudding basin generously with softened butter, making sure to cover the bottom and sides. Pour the golden syrup into the bottom of the basin and spread it gently so it forms an even layer. If you enjoy a slightly tangy flavour, stir a little lemon juice into the syrup before adding it. Set the prepared basin aside while you make the sponge batter.

Place the softened butter and caster sugar in a large mixing bowl. Beat them together using an electric hand mixer or wooden spoon until the mixture becomes pale, light, and fluffy. This process incorporates air into the mixture, helping create a soft sponge texture. Scrape down the sides of the bowl occasionally to ensure the butter and sugar are evenly combined.

Add the eggs one at a time, beating well after each addition. This helps the mixture remain smooth and prevents it from separating. Stir in the vanilla extract and mix until combined. If the batter looks slightly curdled, don’t worry; adding the flour gradually in the next step will help bring everything together.

Sift the self-raising flour, baking powder, and salt into the butter mixture. Gently fold everything together using a spatula until no dry flour remains. Add the milk gradually to create a soft, spoonable batter. Avoid overmixing because too much mixing can make the finished sponge dense rather than light and fluffy.

Spoon the sponge mixture carefully over the golden syrup in the prepared pudding basin. Spread the top evenly without pressing the batter down too firmly. Cover the basin with a sheet of baking parchment followed by aluminium foil, making a small pleat in the centre to allow room for the pudding to rise. Secure the covering with kitchen string, then place the basin on a trivet inside a large saucepan containing gently simmering water.

Cover the saucepan with a lid and steam the pudding for approximately 1 hour 30 minutes, checking the water level occasionally and adding boiling water when necessary. The pudding is ready when a skewer inserted into the sponge comes out clean or with a few moist crumbs. Carefully remove the basin and allow it to rest for 5 minutes. Remove the covering, loosen the edges with a knife, and turn the pudding onto a serving plate so the golden syrup flows over the sponge. Serve warm with custard or cream.