Mousse: Melt the chocolate in a bowl over water. Separate egg, whisk white until stiff but not dry. Whisk in caster sugar. Fold in the two egg yolks. Whisk the cream until it just holds its shape. Dissolve the gelatine in 1 tbsp of boiling water. Fold the melted choc, then the cream, Grand Marnier & gelatine into the egg mixture. Chill the mousse in the fridge.
Baskets: Well oil the outside of a 4in diameter glass basin. Make the caramel by placing the cane sugar, tartar and 2 tbsp of water in a small heavy saucepan over a low heat until dissolved. Do not stir. Turn up the heat and boil rapidly without stirring until pale golden. Remove from heat and stand pan in cold water while caramel continues to cook and becomes a shade darker. Drizzle caramel over the basin to form basket (you're aiming for a filigree effect). Return remaining caramel + a little water to low heat to dissolve. Slice Kumquats, remove pips an poach in caramel for 10 mins, cool.
Divide mousse between baskets and top with glazed kumquats. Serve chilled.