In a bowl, whisk together the milk, eggs, sugar, and vanilla extract until everything is well mixed.
Slice the croissants into thirds and arrange them evenly in a large baking dish.
Sprinkle the blueberries evenly over the croissants.
Pour the custard mixture over everything, making sure each croissant piece is coated. Gently press down to help the croissants soak up the custard.
Cover the baking dish and refrigerate for at least 2 hours, or leave it overnight to soak up all the flavors.
Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius).
Bake the blueberry French toast for about 30 minutes or until the top is golden brown and the custard is set.
Take it out of the oven and let it cool slightly.
Serve it warm, drizzled with maple syrup if you like.
You can store any leftovers in an airtight container in the fridge for up to 4 days.