Combine butter, sugar, zest and juice over medium heat stirring well. Boil, stirring occasionally, for a few minutes until it starts to pull away from the edge of the saucepan and darkens slightly in colour. Mix everything else (except chocolate) together. Pour orange caramel over and stir well. Push into a slice tray and bake at 180 deg cel until top browns slightly. Let cool and smear with chocolate. Set in fridge, flip out and cut.