MAKE THE SIMPLE SYRUP
To a pot over medium heat, add the water, sugar, and orange blossom water. Bring to a boil so all the sugar is dissolved and then reduce the heat to low and simmer for 10 minutes. Transfer to a bowl and allow to cool to room temperature so it thickens up.
PREPARE THE PISTACHIO FILLING
To a bowl, add the pistachio flour, sugar, cornstarch, salt, unsalted butter, vanilla extract, eggs, and heavy cream.
Beat together until light and fluffy using a stand or hand mixer.
ASSEMBLE AND BAKE
Preheat oven to 350°F.
Slice the croissants in half to open them up like a book.
Soak both sides in the simple syrup. Be generous with it. You can brush the syrup on or just dunk it for a more efficient method.
Add the pistachio filling across the whole inside. Close the croissant. Add a line of pistachio filling across the top and sprinkle on chopped pistachios. Repeat with the rest of the croissants.
Bake in the oven for 12 to 15 minutes until crispy all around.
Allow to cool for 15 minutes, then dust with powdered sugar, and enjoy!