Put the raspberries and sugar in a blender or food processor, and purée until smooth.
Reserve 1 tablespoon of the crème fraîche for decorating, and the remaining crème fraîche and the vanilla essence in a bowl. Stir in the raspberry purée.
Whisk the egg whites in a separate bowl until stiff peaks form. Using a metal spoon, gently fold the egg whites into the raspberry mixture until fully combined.
Spoon the raspberry fool into individual serving dishes, and chill for at least 1 hour. Decorate with the reserved crème fraîche.