Preheat your oven to 350°F (175°C) and grease a 9-inch pie dish or baking dish with cooking spray or butter.
In a medium bowl, combine the blueberries, granulated sugar, cornstarch, lemon juice, vanilla extract, and salt. Gently toss until the blueberries are well coated.
In another bowl, whisk together the eggs, milk, and cinnamon (if using) until fully combined.
Pour the blueberry mixture into the prepared pie dish, spreading it evenly. Then, pour the egg mixture over the blueberries, ensuring it covers them completely.
Bake in the preheated oven for 35-40 minutes, or until the center is set and the top is lightly golden.
Remove from the oven and allow to cool for at least 15 minutes before slicing. Dust with powdered sugar if desired before serving.