Preheat the oven to 350°F. Grease a 9×13 inch baking dish with butter. Set aside.

Tear the croissants by hand and fill the baking dish with no spaces between the croissant pieces. Scatter the fresh blueberries over the top.

In a large mixing bowl, combine the eggs, milk, heavy cream, sugar, vanilla, cinnamon, and salt. Whisk until completely combined.

Pour the custard over the croissants. Let it sit for 15 minutes or cover with plastic wrap and refrigerate overnight.

Cover the dish with foil and bake for 25 minutes. Remove the foil and bake for an additional 15 minutes or until golden brown and the custard has set.

Once baked, dot spoonfuls of the mascarpone between the croissants, spoon wild blueberries as desired, and drizzle over the maple syrup. Dust with powdered sugar if desired.

Serve while still warm from the oven.