Wash and dry the orange hot. Finely grate the peel of 1/2 orange, peel a piece of the other half thinly and cut into fine strips. Use the orange for other purposes.
Halve the remaining oranges and squeeze them. Remove the pulp from the halves. Squeeze the pulp and collect the juice. Put the orange halves aside.
Soak gelatine in cold water. Separate eggs. Beat 3 egg yolks, sugar and vanilla sugar with the whisks of the hand mixer until foamy. Stir in grated orange peel, juice and liqueur. Squeeze out the gelatine and dissolve at very low heat.
First mix with 2-3 tablespoons of cream and then stir into the rest of the cream. Chill for 5-10 minutes.
Separately beat egg whites and cream until stiff. As soon as the cream starts to gel, fold cream first, then beaten egg whites into the orange cream.
Pour cream into a piping bag with a large perforated spout and squirt into the orange halves. Cover and chill for approx. 1 hour. Decorate with chocolate rolls and orange strips.