This recipe is from The Jubilee Happy Cooker, published in 1987 by the Union of Jewish Women in Durban, South Africa. It came to me by way of my friend Stephen and his mother, Lesley.
The original recipes calls for a sauce made with 1 cup cream, 180 grams butter, 1 cup sugar, and 1/2 cup hot water. I've reduced the amount of sauce by 25%, as I found my pudding couldn't absorb the full quantity. You might want to pour the sauce over the pudding in stages, reserving some in case your pudding absorbs it all (and so that you don't flood it with more than it can take in right off the bat). —Sarah Jampel
Grease an 8- or 9-inch square baking dish and heat the oven to 350° F (180° C). In the bowl of a stand mixer or working with electric beaters, beat the egg with the sugar very well, until the sugar is dissolved and the mixture is pale in color and greater in volume, 2 to 3 minutes on medium speed. Add the jam and mix to combine.
Melt the butter and add the vinegar. Mix it with the milk. In a separate bowl, sift together the flour, baking soda, and salt.
With the mixer running on low, add the milk mixture and the flour mixture, alternating, in 5 parts (starting and ending with the milk).
Transfer the batter to the baking dish, cover with aluminum foil, and bake for 45 minutes, or until the pudding is nicely browned and set. When the pudding is almost done, make the sauce.
For the sauce:
In a small saucepan, melt all of the ingredients together.
Pour over the pudding as soon as it comes out of the oven and wait for the liquid to absorb. Serve warm.