Preheat oven to 375 degrees.
In a large bowl, combine the half and half, 1 tablespoon vanilla extract, 6 large eggs, & 1/4 cup of powdered sugar. In a bowl, combine the cream cheese, Marscapone cheese, 1/4 cup of the powdered sugar, & 1 tablespoon vanilla extract.
Divide and spread the cream cheese/Marscapone cheese mixture on the bottom half of each large croissant and top with the top half. Dunk each stuffed croissant in the half and half mixture, coating thoroughly. Place in a baking dish. It’s ok if they overlap.
Pour the remaining half & half mixture over the stuffed croissants. Cover with foil and refrigerate for at least 2 hours or preferably overnight.
Pour the frozen blueberries over top. (See notes for using fresh blueberries) Bake, covered, for 25-30 minutes. Uncover and bake for an additional 15 minutes. Garnish with powdered sugar, if desired.