Set your oven rack to the middle position and heat the oven to 350 degrees.
1 small loaf challah or brioche
Use your hands to tear the bread into small pieces until you have 6 cups total (save the rest of the bread for another use). Spread the bread pieces out into an even layer on a rimmed baking sheet.
Bake in the preheated oven until the bread pieces are dry, crisp, and beginning to turn golden, 10 to 15 minutes.
2 cups whole milk, ½ cup granulated sugar, ½ teaspoon ground cinnamon, Pinch table salt, 2 tablespoons unsalted butter, cut into 2 pieces
Meanwhile, in a large liquid measuring cup or microwave-safe bowl with a spout, whisk together the milk, sugar, cinnamon, and salt, then add the butter. Microwave until the butter is just melted, 3 to 5 minutes, stopping and stirring with a spoon a few times as the mixture heats. Remove from the microwave and set aside to cool for at least 5 minutes.
When the bread is ready, use oven mitts to remove the baking sheet from the oven and place it on the stovetop or a cooling rack. Let cool for 5 minutes.
Cooking spray, 1 ¼ cups fresh or frozen blackberries
Spray the inside of an 8-inch square or other 2-quart baking dish with cooking spray. Transfer the toasted bread pieces to the greased baking dish. Sprinkle evenly with the blackberries.
4 large eggs, 1 teaspoon vanilla extract
Add the eggs and vanilla to the cooled milk mixture and whisk until well combined.
1 tablespoon turbinado sugar
Pour the milk mixture evenly over the bread and berries in the baking dish. Use the back of the spoon to lightly push the bread and berries down until they’re covered in the custard. Sprinkle with turbinado sugar, if using.
Bake in the preheated oven until the bread pudding is puffed and deep golden brown on top, 40 to 45 minutes.
Use oven mitts to remove the baking dish from the oven and place it on the stovetop or a cooling rack. Let cool for at least 15 minutes. Serve warm or room temperature.