Preheat the oven to 180 degrees. Line the base of a 20cm round tin with baking paper.
Sift the flour, cocoa, baking soda and sugar into a bowl.
Pour in the melted butter, warmed milk and vanilla essence and mix a little with a wooden spoon.
Add the vinegar and mix until it is a smooth batter.
Pour mixture into the prepared tin and bake for 45-55 minutes, or until a skewer comes out clean.
Leave in the tin for 15 minutes before turning out on to a rack to cool.
Ice with either chocolate icing or ganache.
Chocolate Icing
Sift the icing sugar and cocoa into a bowl. Add the butter, and mix in a little boiling water at a time, until the icing is smooth and spreadable.
Ganache
Heat the cream until just starting to boil. Take off the heat and whisk the dark chocolate into the cream unitl smooth. Allow to cool a little before pouring over the cooled cak