In a large bowl, whisk together the eggs, milk, melted butter, maple syrup, and vanilla.
Spray an 8x8-inch casserole dish with cooking spray. Add the bread and 1 cup of blueberries (reserve 1/2 cup). Pour the egg mixture evenly over the bread and blueberries and toss to make sure every piece of bread is moist. Spread the remaining ½ cup of blueberries on top.
Cover and refrigerate for at least 3 hours and up to 24 hours, if not freezing.
Preheat the oven to 350°F, while the casserole sits on the counter to warm slightly. Bake, uncovered, for 50-60 minutes or until golden brown on top and set in the middle. (If it’s browning too much on top, tent the casserole with foil.)
Make the Vanilla Glaze. In a small bowl, stir together the milk, vanilla, and salt. Add the powdered sugar and whisk until a thin, white glaze forms.
Let the casserole cool for about 10 minutes. Then, use a spoon to drizzle the glaze over the casserole (you don’t have to use all of it). Serve warm, with more maple syrup, if desired.