Preheat the oven to 375ºF (190ºC). Set 8 small panettone molds on a rimmed baking sheet.
In a small bowl, whisk together the flour, baking powder, and baking soda.
In a bowl of a stand mixer fitted with the paddle attachment, or by hand in a medium bowl, beat the sugar, eggs, and salt on medium speed until the batter is thick and pourable, like a melted milkshake, about 2 minutes. Reduce the speed to low and beat in the jam until incorporated. Then mix in the melted butter and vinegar.
Add one third of the milk, then half of the flour mixture. Then add another third of the milk, then the rest of the flour. Finally, add the rest of the milk and stir until smooth. If there are large lumps, whisk it briefly, just enough until the batter is smooth.
Pour the batter into the 8 molds, which will fill them a little more than halfway up. Bake the cakes until they are deeply bronzed across the top, 35 to 40 minutes. The cakes need to be well-baked, or as Jessica says, "...almost overbaked - in order to soak up the filling."