For the clementine marmalade: Put the clementines (with the skin on) in a pot and cover with water. Bring to a boil, then reduce the heat and simmer until tender, about 30 minutes. Drain and let cool.
Thinly slice the clementines and remove any seeds. Return to the pot along with the sugar and the lemon zest and juice. Simmer over low heat until the peels are translucent and candied, about 1 hour.
For the pastry puffs: Combine the milk, butter, granulated sugar and salt in a heavy saucepan over medium-high heat. Bring to a boil. Remove from the heat and quickly add the flour, stirring with a wooden spoon. Return to the heat and cook, stirring constantly, until the dough pulls away from the sides of the saucepan and forms a thick, smooth ball, 1 to 2 minutes. Remove from the heat. Beat in 4 eggs, one at a time, with the wooden spoon until smooth and thick. When you put a small amount of dough between two fingers and pull your fingers apart, two peaks should form on the dough.
Preheat the oven to 425 degrees F (220 degrees C). Line a baking sheet with parchment paper.
Transfer the dough to a piping bag. Pipe mounds on the prepared baking sheet, spacing them about 2 inches apart. Beat the remaining egg with a bit of water for the egg wash, and use a pastry brush to gently brush it over the tops of the dough. Gently flatten any peaks on top of the mounds. Sprinkle with the crushed pistachios and brown sugar.
Bake for 15 minutes. Reduce the oven temperature to 350 degrees F (180 degrees C) and bake until the pastry puffs are golden and dry inside, 15 minutes more. Turn the oven off. Prop the oven door open so that it is slightly ajar, and let the pastry dry in the oven for 15 minutes. Remove from the oven and let cool on a wire rack.
For the custard cream: Whisk together the cream and 1/2 cup sugar in a saucepan. Add the scraped vanilla and bean. Bring to a boil, stirring to dissolve the sugar, and remove from the heat.
Beat the egg yolks and remaining 1/2 cup sugar until thick enough to hold a ribbon on the surface when you lift up the whisk. Slowly beat in the hot cream mixture. Return the mixture to the saucepan and cook, stirring constantly, until it coats the back of a wooden spoon, about 5 minutes. Strain into a bowl and press a piece of plastic wrap directly onto the surface of the custard to prevent a skin from forming. Refrigerate until cold, about 1 hour.
Stir 1 cup (250 ml) clementine marmalade and the whole pistachios into the cold custard. Cover with plastic wrap and freeze until firm, about 4 hours.
For the chocolate sauce: Bring the cream just to a simmer in a small saucepan over medium heat. Add the chocolate, melt it and stir with a wooden spoon until smooth. Keep warm.
To serve, split the pastry puffs in half, fill the bottom half with a scoop of the frozen custard and replace the top. Place on a plate and drizzle with warm chocolate sauce. Sprinkle with pistachios. Serve immediately.
Cook's Note: The pastry puffs can be frozen. Thaw them and then reheat in a 350 degree F (180 degree C) oven for 5 to 10 minutes, or until crisp. Cool before filling.
Choux pastry rises due to the egg content and steam. Therefore, it is essential that the oven is hot when you add the pastry puffs. Without this initial burst of steam they will not rise properly or dry out; they will stay flat and become soggy.