Generously spray a 1.5-quart casserole dish with cooking spray. Evenly spread the sourdough pieces on the bottom of the pan. Don't stress if there is some overlap. In a large bowl, whisk together the eggs, milk, heavy cream, sugar, and vanilla. Pour the egg mixture evenly over the bread. Cover tightly with plastic wrap and chill for several hours or overnight.

In a separate bowl, combine the flour, brown sugar, cinnamon, salt, and butter pieces. Using a pastry cutter or your fingers, mix the ingredients together until they resemble coarse sand. Cover the mixture tightly with plastic wrap and chill until ready to use. The next day, preheat the oven to 350 degrees. Evenly sprinkle the dry ingredients over the bread. Bake the cinnamon toast until lightly golden and crisp, about 1 hour. Serve immediately with butter and maple syrup, if desired.