Preheat the oven to 180C and line a small loaf tin/ 6” round cake tin, greasing well. Set aside.
Stir the vinegar into the milk and set aside.
Beat the butter and condensed milk together until soft and creamy.
Beat in the sugar and vanilla extract.
Whisk in the vinegar & milk mixture.
Sift the dry ingredients into this wet mixture and gently fold until combined. The batter will be thick.
Pour the batter into the prepped cake tin and smoothen the top.
Bake for 18-20 minutes until the tops are golden and a skewer inserted in the centre comes out mostly clean, with a few moist crumbs attached.
Let the sponge cake cool down on a wire rack before slicing and serving!