Generously grease a 9x13 inch baking dish with the softened unsalted butter. Spread the cubed bread evenly across the bottom of the dish. If the bread is very fresh, allow it to sit out for an hour or two to dry slightly.

Sprinkle the 2 cups of blueberries evenly over the layer of bread cubes.

In a large mixing bowl, whisk the 8 large eggs vigorously until slightly frothy. Add the whole milk, granulated sugar, brown sugar, vanilla extract, cinnamon, nutmeg, and salt. Whisk thoroughly until the sugars are completely dissolved and the mixture is well incorporated.

Carefully pour the custard mixture slowly and evenly over the bread and blueberries in the baking dish. Gently press down on the bread cubes with a spatula to help them absorb the custard fully.

Cover the baking dish tightly with plastic wrap. Refrigerate the casserole for a minimum of 4 hours, or ideally, overnight (up to 12 hours). This step is crucial for the best texture.

The next morning, remove the casserole from the refrigerator. Preheat your oven to 375°F (190°C). Allow the casserole to sit on the counter for 20 to 30 minutes to come closer to room temperature while the oven preheats.

Remove the plastic wrap and place the casserole in the preheated oven on the center rack. Bake for 45 to 55 minutes, or until the top is golden brown, puffed slightly, and the center is set (a knife inserted into the center should come out clean of wet custard).

Remove from the oven and allow it to cool slightly for 10 to 15 minutes before serving. Dust generously with powdered sugar and serve warm with a drizzle of maple syrup.