Put the chocolate, butter and cream and liqueur or vanilla if using into a double boiler and melt gently . Alternatively put in the microwave for 1 minute then stir till melted. Set aside
Meantime using a very clean bowl and whisk beat the egg whites till frothy and white then add the sugar 1 spoon at a time. Beat till tripled in volume and glossy.
Fold the whites into the chocolate in three batches. Divide into 6 small glasses or ceramic dishes. Chill overnight or for several hours
Smash up a packet of Oreo cookies in the processor or in a plastic bag with something heavy, leaving a few lumps and clumps. Sprinkle the bits onto the top of the mousses. ( setting a little aside)
Put the sprigs of basil or herb into each plant pot.
Put on a tray and sprinkle with some leftover crumbs to make it look like they have just been potted.