For the choux pastry, preheat the oven to 180C/350F/Gas 4. Line two baking trays with parchment paper. Place a 23cm/9in metal ring on one of the lined trays.
Put 125ml/4fl oz of water, the milk, butter, salt and caster sugar in a saucepan set over a high heat for one minute, stirring with a spatula. Turn off the heat and, stirring all the time, quickly add the flour until the mixture is very smooth.
When the mixture is smooth, return the pan to the heat and stir with the spatula for one minute. The paste will begin to poach and some of the water will evaporate. Don’t let the paste dry out too much, or it will crack during cooking. Immediately remove from the heat and add the eggs, one at a time, mixing in with a spatula between each addition.
Stir well with the spatula until the eggs are thoroughly combined and the paste has a very smooth texture. It is now ready to use.
Carefully spoon the mixture into a large piping bag fitted with a plain 1cm/½in nozzle.
For the gâteau Saint Honoré, use half of the choux pastry to pipe 12 small buns on one of the prepared baking trays. Pipe the remaining choux pastry into the metal ring on the other baking tray. Transfer the baking trays to the oven and cook for 10-15 minutes, or until risen and golden-brown. Set aside to cool.
Whip the double cream in a bowl until firm, split the vanilla pod and add the seeds to the cream. Fold the custard into the cream together with two tablespoons of the sugar and a dash of the orange liqueur. Set aside.
Heat the remaining sugar in a non-stick pan over a medium heat and melt to form a caramel. Grease a baking tray with butter.
Carefully dip each choux bun into the caramel and then place on the greased baking tray to cool. If the caramel begins to set, warm it gently.
Remove the ring from the pastry base and drizzle the remaining liqueur over the middle. Using an ice cream scoop or a tablespoon warmed in hot water, place half of the custard and cream mixture in the centre of the base.
Fill a piping bag fitted with a small plain nozzle with the remaining cream mixture. Use a small, sharp knife to cut small holes in the underside of the choux buns and fill them with the cream mixture. Arrange the buns around the edge of the flan. Garnish one side with roses and twigs and serve.