Heat the butter and chocolate in a pan until just melted.
Remove from the heat and add the brewed coffee. Set to one side to cool.
Whisk the eggs, brown sugar and vanilla together for about 30 seconds.
Add the cooled chocolate mixture and whisk it in, just until everything is nicely combined.
Add the flour, cocoa and vanilla sugar and once again, whisk just until everything is well combined.
Pour the batter into the tin and bake for around 18-20 minutes, just until the middle is firm.
Remove from the oven and allow to cool completely.