Pre-heat the oven to 170°C (325°F) and grease (or line) a large rectangle or square springform pan (mine was 27cm x 17cm / 10in x 7in).
Slowly heat the chocolate and butter in a pan until just melted, then add the coffee/water and milk. Leave the mixture to cool for about 10 minutes.
In a large bowl, whisk the sugar with the egg, then quickly beat in the cooled chocolate mixture.
Sift the vanilla sugar, flours and cocoa together and stir into the wet ingredients until well combined.
Pour into the prepared tin and bake for around 20 minutes (until an inserted skewer comes out clean). Remove from the oven and allow to cool (leave the oven on).