Orange custard filling:
Slice the vanilla pod in half and scrape out the seeds. Place the pod and the seeds into a medium sized pan.
Add the milk and bring to the boil.
Meanwhile, in a large bowl, mix together the egg yolks, sugar and cornflour.
Once the milk has boiled, pour into the egg mixture and mix well with a wooden spoon, stirring continuously until the mixture thickens. Remove the vanilla pod, cover with wet greaseproof paper or cling film and set aside to cool.
In a separate large bowl, add the whipping cream, orange zest and orange blossom flower, if using, and whisk to soft peak.
Fold the cream into the cooled custard.
Spoon the orange cream into a piping bag with a medium-sized nozzle and keep in the fridge until needed.
Preheat the oven to 220c/190c/gas mark 7.
Cut the butter into cubes
Place the water, butter and sugar into a medium sized pan and bring to a simmer. Remove from the heat and tip all the flour into the pan in one go. Beat the mixture for 2 minutes or until it starts to come away from the side of the pan.
Return the pan to the heat for a minute and allow the flour to cook through, stirring occasionally.
Remove the pan from the heat and cool for a further 5 minutes. Add the eggs, one at a time, beating the mixture well between each addition.
Spoon 12 equal sized rounds onto a baking tray that has been lined with greaseproof and push down the peaks with a wet finger. Each one should be slightly bigger than a golf ball.
Bake the choux buns in the oven for 20 minutes until golden, then remove from the oven.
Using a sharp knife, pierce a hole in the bottom of each choux bun to release the steam and place back on the baking sheet on its side. Return to the oven for a further 5 minutes to allow the choux bun to dry out completely.
Remove choux buns from the oven and place on a wire rack.
Meanwhile, chop the butter into cubes. In a heatproof glass bowl, combine the butter and chocolate and melt over a pan of simmering water. Stir continuously until the butter and chocolate are melted and well combined.
To assemble the choux buns, using your piping bag to fill each bun with orange cream through the hole in the bottom. Dip the filled bun top side down into the chocolate and place onto a baking sheet. Repeat until all the buns are done.
Finally sprinkle the buns with chopped pistachio nuts and let the pinching begin.