Preheat your oven to 350°F (175°C). Using a pastry brush or paper towel, spread the 1 tablespoon (14 g) unsalted butter, softened, for greasing the baking dish evenly over the bottom and sides of a 9 x 13-inch (23 x 33 cm) baking dish. Make sure the corners are coated so the baked pudding releases more easily. Set the prepared dish aside while the oven heats.

Place the 8 cups (510 g) slightly stale brioche or challah bread, cut into 3/4-inch (2 cm) cubes, evenly in the prepared baking dish. Keep the bread in a loose, even layer rather than packing it down. The cubes should look slightly dry on the outside but still have some softness in the center.

Crack the 6 large eggs (approximately 300 g without shells), at room temperature, into the large mixing bowl. Add the 3/4 cup (150 g) granulated sugar and 1/3 cup (70 g) packed light brown sugar. Whisk for about 1 to 2 minutes, until the yolks are fully broken up and the mixture looks smooth and evenly combined. Break apart any visible clumps of brown sugar.

Add the 1 can (15 ounces/425 g) plain pumpkin puree, at room temperature, 2 cups (480 ml) half-and-half, at room temperature, 1 cup (240 ml) whole milk, at room temperature, 2 teaspoons (10 ml) pure vanilla extract, 1 teaspoon (2.5 g) ground cinnamon, 1 teaspoon (2 g) ground ginger, 1/4 teaspoon (0.5 g) ground nutmeg, 1/4 teaspoon (0.5 g) ground cloves, and 1/2 teaspoon (3 g) fine salt. Whisk for about 2 minutes, until the custard has a uniform orange color and no large streaks of pumpkin remain. It should be smooth and pourable.

Slowly pour the pumpkin custard over the bread, moving across the entire baking dish so the liquid reaches the corners. Gently press the exposed bread cubes into the custard with a silicone spatula. Leave the mixture at room temperature for 30 minutes, gently pressing the bread down again halfway through. Don't worry if some pieces float at first. By the end of the soaking period, the bread should look swollen and thoroughly moistened, with much of the custard absorbed.

Place the baking dish on the center rack of the preheated 350°F (175°C) oven. Bake for 45 to 50 minutes, until the edges are lightly golden, the custard looks set, and the center has only a slight wobble when the dish is gently moved. Check that the center reaches at least 160°F (71°C) with an instant-read thermometer. If the center still looks liquid or has not reached that temperature, continue baking until it is set and reaches the safe temperature.

Carefully remove the baking dish from the oven using oven mitts and place it on a wire cooling rack. Let the pumpkin bread pudding rest for 10 minutes. The surface may settle slightly as it cools, and that's normal. The custard should become a little firmer while remaining moist and tender inside.

After the 10-minute rest, divide the bread pudding into 12 portions using a serving spatula. Lift each portion gently to keep the soft bread and custard together. Serve while warm, when the top is lightly golden and the center is creamy but fully set.