Pre-heat the oven to 175°C (350℉).
Grease a 9 inch (24cm) tin spring-form tin (you can use a smaller one but you will need to adjust the baking times).
Add the butter and chocolate to a pan and slowly heat until just melted.
Add the coffee (or water) and stir to combine. Leave the mixture to cool for a short while.
In a bowl, whisk the eggs and sugars together.
Pour in the chocolate mixture and stir until everything is combined.
Add the flour, cocoa and vanilla sugar and once more, stir, just until everything is well incorporated.
Pour into the prepared pan. Bake between 15-25 minutes (see note 2).
When ready, remove from the oven and allow to cool completely (keep it in the springform).