1. The day before, prepare the brioche dough. Put the flour, salt, powdered sugar, yeast, and eggs in the bowl of a mixer.
2. Mix for about 5 minutes.
3. Add the softened butter and lemon zest.
4. Mix again for 5 minutes.
5. Cover the dough with a cloth and refrigerate overnight.
6. The next day, roll out the dough to about 1.5 cm thick and 20 cm in diameter. Place it in a cake mold.
7. Let the dough rise, covered with a cloth, at room temperature for 2 hours.
8. Prepare the pastry cream.
9. Bring the milk to a boil with the split vanilla bean.
10. Preheat the oven to 180°C (350°F).
11. In a bowl, whisk the egg yolks and powdered sugar until pale.
12. Add the flour, then pour the hot milk over it while whisking vigorously. Mix well.
13. Add 2 tablespoons of orange blossom water and mix.
14. Cook the mixture in a saucepan for 3 minutes, stirring constantly, once it comes to a boil.
15. Once the cream is cooked and thickened, pour it onto a tray, cover it with plastic wrap (to retain moisture), and let it cool quickly.
16. When the brioche has risen after 2 hours, brush it with a beaten egg yolk diluted in a little water and sprinkle with coarse sugar. Bake in a preheated oven at 180°C (350°F) for about 30 minutes.
17. Once the brioche is cooked, remove it from the oven and let it cool, then cut it in half horizontally.
18. Prepare the whipped cream. Put the mixer bowl, whisk, and heavy cream in the freezer for 5 minutes.
19. Whip the cream at medium and steady speed, when it is halfway whipped, add the powdered sugar, increase the speed until the cream is firm. Keep refrigerated.
20. When the pastry cream has cooled, mix it with the whipped cream.
21. Using a piping bag, pipe the cream onto the brioche disc, then close it with the other disc, sprinkle with powdered sugar, and refrigerate until ready to serve.