Whisk the mascarpone with the icing sugar until smooth and set aside. Put the leaf of gelatine in a small bowl of cold water for a few minutes to soften.
Heat 25ml of the cream in a small saucepan and once hot, remove from the heat, squeeze the water from the gelatine leaf and dissolve in the hot cream. Set aside.
Meanwhile whisk the remaining 100ml cream in a bowl to soft peak.
Pour the hot cream over the melted white chocolate and beat well to combine. Beat the chocolate into the mascarpone, then fold in the whipped cream.
Add the orange zest and most of the chopped stem ginger and mix through, reserving a little stem ginger to garnish. Leave the cream to set for at least one hour, then beat well in a bowl until smooth. Keep chilled until ready to fill the choux.
For the Craquelin Topping:
Rub all of the ingredients together in a bowl until the dough comes together. Roll the dough as thin as possible (2mm) between two sheets of parchment paper and place the sheet in the freezer for a minimum of one hour.
Cut out small circles using a round cutter (3cm) when frozen and place back in the freezer until the choux is ready to be baked.
For the Choux Buns:
Preheat the oven to 200C or 180C fan and line a baking tray with parchment paper.
Put the water and milk in a saucepan with the salt, sugar, and butter over a medium heat. Bring to a boil.
Add the flour gradually, reduce the heat to low and mix over the heat for 1-2 minutes to cook out the starch.
Remove from the heat and put the dough in a stand mixer fitted with a beater attachment. Beat on a medium speed for 2 minutes to cool.
Once cool, add the egg, and mix until a dropping consistency is reached. Put the choux paste into a piping bag fitted with a small round nozzle. Pipe the choux (3cm in diameter) onto the baking tray, leaving a few centimetres between each. Press a frozen craquelin disc on the top of each choux.
Bake the choux in the preheated oven for 7 minutes, then reduce the heat to 180C or 160C fan and continue to bake for a further 13 minutes.
Remove from the oven and set aside to cool completely.
The unfilled choux can be frozen at this point or filled with the cream to serve.
Put the ginger, white chocolate and orange cream in a piping bag fitted with a small star nozzle.
Trim the tops off the choux, and fill with cream. Place the tops on top of the cream and pipe more cream on top. Garnish with the reserved chopped stem ginger and serve.