Pre-heat the oven to 170°C (338℉) and grease a fairly large square or rectangle spring-form baking pan (mine was 27cm x 17cm / 10in x 7in).
Slowly heat the chocolate and butter in a pan until just melted.
Mix the coffee (if using) together with the milk, and pour into the chocolate mixture. Let it cool for about five minutes.
In a large bowl, whisk the sugars with the eggs, and if you're using vanilla essence rather than vanilla sugar, add it now.
Add in the cooled chocolate mixture, and whisk just until combined.
Mix the flour and cocoa together, then quickly whisk into the wet ingredients (don't over work it, just until everything is combined).
Pour into the prepared tin and bake for around 20-25 minutes (until an inserted skewer comes out clean). Remove from the oven and allow to cool completely (leave the cake in the pan).