Preheat the oven to 200C/180C Fan/Gas 6. Spray a 6 hole muffin tray with oil/vegan butter spray.
In a large bowl, combine the milk and vinegar and let sit a few minutes. Add the butter, vanilla and sugar and mix.
Add in the flour, baking powder and baking soda. Stir together until just combined.
Spoon about 1 tbsp batter into each muffin hole. Add a teaspoon of jam in the middle of each, before topping with the remaining batter. Bake for 20-25 minutes, until golden brown. Remove from the oven and let cool completely.
Once cool, remove each muffin from the tin (gently ease out). Then melt the butter and put the sugar in a large bowl/plate. Brush the butter over each muffin before dunking and coating in the sugar!