Preheat the oven to 340°F.
Line and lightly grease three 6-inch baking tins.
Mix together the milk, espresso, and vinegar, then set aside for 10 minutes.
Once thickened, pour into a large mixing bowl and add the remaining wet ingredients.
Sift in the dry ingredients.
Gently mix until just combined.
Pour the batter into the tins and bake for 25-30 minutes, or until well risen and cooked through.
While the cakes are cooling, make the caramel sauce and buttercream.
For the caramel, combine all the ingredients.
In a stand mixer, cream the butter on medium-high until light and fluffy (about 5 minutes), scraping down the bowl and paddle a few times. Add the powdered sugar bit by bit, scraping down the bowl and paddle between intervals. Then add the caramel sauce and mix on low until fully incorporated, about one minute.
Layer the cooled cakes with the frosting and caramel.