Preheat the oven to 180℃ (350℉). Ovens are temperamental, so if you know yours needs a bit more (or less) of a push, feel free to adjust.
In a large mixing bowl add your first 4 ingredients - butter, sugar, jam and egg. Beat until it lightens and gets a bit fluffier. If you only have a fork, the fluffy part isn’t make-or-break.
In a large measuring cup or a bowl, add milk, then vinegar and vanilla essence. Whisk lightly.
In another measuring cup/bowl, add bicarb to flour and give it a quick stir.
Alternating between the milk mixture and the flour, add to the large mixing bowl and mix well as you go along.
Pour into a lightly greased oven-proof dish (ours was 30x15cm, but it could definitely have been a bit smaller).
Bake for 45 minutes, or until a knife comes out with spongy crumbs stuck to it (it’s not likely to come out clean) rather than uncooked batter. The pudding will stay in its mold and rest for a bit once the sauce is poured over.