In a pot over medium heat, combine the water, sugar, and orange blossom water.
Bring the mixture to a boil until the sugar is completely dissolved, then reduce the heat to low and let it simmer for 10 minutes.
Transfer the syrup to a bowl and let it cool to room temperature until it thickens.
**Preparing the Pistachio Filling:**
In a bowl, mix together the pistachio flour, sugar, cornstarch, salt, unsalted butter, vanilla extract, eggs, and heavy cream.
Beat the mixture until it's light and fluffy using a stand or hand mixer.
**Assembly and Baking:**
Preheat your oven to 350°F.
Slice the croissants in half to open them up.
Generously soak both sides of the croissants in the simple syrup.
Spread the pistachio filling evenly across the inside of each croissant and then close them.
Add a line of pistachio filling on top of each croissant and sprinkle chopped pistachios.
Bake in the oven for 12 to 15 minutes until they're crispy all around.
Let them cool for 15 minutes, then dust with powdered sugar before serving.
Notes
You can make pistachio flour by blending raw or roasted pistachios in a food processor until they resemble sand. Be careful not to over-blend, or they might turn into a paste.