Spread half the bread cubes into a greased 9x13 inch baking dish and top with half of the blueberries. Repeat with remaining bread and berries.
In a large mixing bowl, whisk together eggs, milk, cream, sugar, vanilla, lemon zest, and salt until thoroughly combined. Pour custard mix over the bread and gently push down with a spatula to ensure the bread soaks up as much as possible. Let soak for at least 6 hours or, ideally, overnight.
Preheat the oven to 350F and remove the dish from the refrigerator.
Sprinkle the cubed butter (and raw sugar, if using) over the top and then bake for 45 minutes, or until a knife inserted in the middle comes out clean.
Dust with powdered sugar and serve with maple syrup.
Brioche, challah, or any rustic-style loaf works best
If using frozen berries, be sure to keep them frozen while completing the recipe for best results