Add all ingredients except the dark chocolate and decorative goji berries into a food processor and blend until a slightly sticky dough forms.
Spread the dough on a parchment- lined 20 cm x 20 cm rectangular tin.
Place another sheet of parchment paper on top and press down firmly until the surface is level ( I use a little rolling pin).
Take off the parchment paper on top and refrigerate.
Meanwhile, melt the dark chocolate in a bain-marie ( a pot of simmering water, with a heatproof bowl on top of it). When you are short on time melt the chocolate in a small bowl in the microwave. Don't leave it unattended, heat it slowly in 10-20 second frequencies and keep checking.
Take the dough out of the fridge and pour the melted chocolate on top.
Quickly decorate with goji berries before the chocolate sets.