Pre-heat the oven to 180°C (230°F) and line or grease two 8 inch pans.
In a large bowl, sift the flour, vanilla sugar, cocoa, baking powder and baking soda. Stir in the white and brown sugar.
In a separate bowl, whisk the milk, oil and eggs until well combined.
Add the wet ingredients to the dry and fold together. Add the chopped chocolate.
Finally, slowly add the coffee or hot water (it should be recently boiled) and stir until everything comes together. Don't overmix, there will still be a few lumps left in the batter (also note that the batter is supposed to be runny, so don't panic!).
Divide between the two prepared pans as equally as you can and then bake side by side for around 30-35 minutes. The cakes are ready when they bounce back when gently pushed on the top, and an inserted skewer comes out clean.
Remove from the oven and leave the cakes in the pans for around 15 minutes before turning out onto a wire rack to cool completely before decorating.