Prepare gratin pan or 9x9 baking dish with cooking spray.
Add cubed croissant pieces to dish and top with blackberries. Make sure to tuck some blackberries inside the croissant!
Beat cream cheese and sugar in a stand mixer or with a hand mixer.
Add the eggs, one at a time and mix until just combined.
Add vanilla and slowly add milk while continuing to mix ingredients.
Pour the custard over the croissant pieces and let rest for 20 minutes.
Meanwhile, heat the oven to 350 degrees. Bake for 35-40 minutes until golden brown.
If at 30 minutes the tops are too golden, place a sheet of foil over the baking dish.
Sprinkle with powdered sugar and serve warm.