Heat 225ml water and 100g caster sugar in a pan.
Stir until sugar melts and boil until slightly reduced.
Remove pan from heat and allow to cool.
Add lavender flowers to the cooled syrup.
Line six ramekins with cling film and warm 450ml cream with vanilla sugar and orange zest.
Strain the cream mixture through a sieve into a bowl.
Dissolve gelatine in two tablespoons of water over low heat.
Combine the warm cream mixture with the dissolved gelatine and cool.
Whip the remaining 150ml of cream softly and fold into the mixture.
Pour mixture into the ramekins and refrigerate.
Chill for at least two hours until firmly set.
Unmould panna cottas onto plates and drizzle with lavender syrup.