Lightly spray 9x13 baking dish with cooking spray. Set dish aside.
Slice croissants in half, cutting through middle of croissants to create croissant buns. Detach halves completely and set aside.
Add eggs, half and half, vanilla extract, cinnamon, salt, and maple syrup to large mixing bowl. Whisk ingredients together until fully combined.
Submerge one croissant half in egg wash until completely soaked, then place croissant half cut-side down in greased casserole dish. Repeat until all croissant halves have been dipped in egg wash and placed in casserole dish. Overlap croissant halves slightly as needed to fit in dish.
Cover baking dish tightly with lid or food-safe plastic wrap. Place covered baking dish in refrigerator and chill at least 2 hours, but no more than 24 hours.
Remove baking dish from refrigerator 30 minutes prior to baking and let stand at room temperature. While casserole comes to temperature, preheat oven to 375° Fahrenheit.
When oven is preheated, discard plastic wrap and place baking dish in oven. Bake casserole, uncovered, 25 to 35 minutes, or until croissants are golden brown and heated through completely.
Remove casserole from oven. Let stand 5 minutes, then dust top with powdered sugar and add fresh fruit and whipped cream if desired. Serve immediately with pure maple syrup or bourbon-vanilla maple syrup.