Soak gelatin. Beat egg whites until stiff, chill. Mix the vanilla pulp with the yogurt, powdered sugar and lemon. Stir in the dissolved gelatine evenly. First fold in the double cream, then fold in the egg whites. Chill the yoghurt mixture overnight.
For the sauce, squeeze the lemon, julienne the lemon peel. Bring the bowl, sugar and 150 ml water to the boil. Add tarragon and simmer covered for 10 minutes. Remove from heat, stir in 4 tablespoons of lemon juice and the plum water and set aside.
Quarter the strawberries and add them to the syrup. Marinate for 1 hour. Serve the mousse socks with the strawberries and juice.