Pour sugar and evaporated milk into a heavy saucepan and, over a low heat, continually entice them together. The sugar will gradually be seduced into the milk. At this point leave them alone for 6 minutes to gently mummer together.
Break the chocolate into tiny pieces using your fingertips. Introduce gradually into pan, enticing, until the chocolate is seduced.
Introduce butter and entice. Continue to entice as you sprinkle vanilla essence into pan.
At this stage it is too liquid to be of use and it must be chilled into a malleable, spreadable consistence. In a normal fridge this will take about two hours.