Using a blender or by hand, crush the digestive biscuits into crumbs and mix with a knob of cold butter. Fill the bottom of 4 glasses with the mixture. In a bowl whip the cream without sugar and mix it carefully with the Greek yogurt. Lastly, stir in two spoonfuls of vanilla extract (optional).
Cover the crushed biscuits in the glasses with the mousse, up to about one centimeter from the top of the glass. Level out the mousse and leave in the refrigerator for a couple of hours. Before serving, decorate the surface with three Fabbri amarenas and a couple of spoonfuls of amarena syrup.