Pre-heat the oven to 175°C. 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.
Whisk the eggs and sugars together then 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, depending on how well cooked you want your middle. Baking for 15 minutes will produce a very molten, runny centre. 25 minutes will give you a firm, brownie-like texture.
When ready, remove from the oven and allow to cool completely.
For the topping: mix all of the ingredients apart from the butter, in a pan. Let the mixture cook until it starts to thicken (around 15 minutes). Remove the pan from the heat and beat in the butter until a nice fudgy texture is achieved. Allow the mixture to cool (and thicken for a while) before pouring over the cake. Allow to set completely (I usually leave it in the fridge overnight). When ready to eat, slice using a hot knife. if you want a gooey, soft fudgy topping, don't keep the cake in the fridge.