Put 12.3 oz of sugar in a saucepan, add 3.5 fl oz of water and allow to caramelise. When the syrup takes on a uniform amber colour, add 1.1 fl oz of vinegar and 7.1 oz of cream. Cook again for 1 minute and let the butterscotch cool down for about an hour.
Put a generous layer of butterscotch on the bottom of a glass, heat 5.3 fl oz of milk without boiling it and emulsify it with a hand blender or a piston coffee maker. This will result in a soft foam.
Prepare the coffee and, once ready, pour the milk into the glass first and then the equivalent of an espresso cup.