Start by setting your oven to 300°F. Cut the 18-ounce Italian fruit panettone into rough 1.5-inch cubes with a serrated knife. Keep the pieces a little uneven because the smaller bits will turn crisp while the larger ones stay soft. Spread the cubes in one loose layer on a large rimmed baking sheet. Toast for 8 to 10 minutes and turn them once halfway through. Then remove the sheet and let the bread cool for 5 minutes. Don't let it turn dark brown. The pieces should feel slightly dry on the outside but still soft when pressed in the middle. You'll smell the orange peel as they warm. Toasting too long will make the edges hard instead of tender. A brief dry toast helps the bread absorb custard without falling apart.

While the bread cools, crack 6 large eggs into a large mixing bowl and whisk for 45 seconds until the yolks and whites blend. Add 1/2 cup granulated white sugar and keep whisking for another minute. Next pour in 2 cups whole milk and 1 cup heavy whipping cream. Add 2 teaspoons pure vanilla extract and 1 teaspoon ground cinnamon with 1/4 teaspoon ground nutmeg and 1/4 teaspoon fine sea salt. Whisk for 60 seconds until the sugar starts to dissolve. Keep everything near 68°F and don't heat the mix. The custard should look pale yellow with tiny cinnamon flecks spread through the liquid. No thick streaks of egg should remain. If you stop whisking too soon, some bites may have a firm eggy texture. A smooth cold custard helps the pudding bake with an even creamy center.

Raise the oven temperature to 350°F. Rub 1 tablespoon softened unsalted butter over the bottom and sides of a 9-inch square baking dish. Transfer the toasted panettone cubes into the dish and spread them in loose uneven layers. Pour the vanilla custard slowly over the bread and make sure you reach the corners. Then press the raised cubes gently with the back of a large spoon. Let the mixture soak for 20 minutes and press the top again after 10 minutes so the dry pieces absorb more liquid. The bread should look plump and moist with a little custard still visible between the cubes. Some golden edges will stick out above the liquid. Don't leave the mixture soaking for hours on the counter because the bread can break down. A 20-minute soak gives the centers time to soften while the tops keep their shape.

Place the filled baking dish on the middle oven rack at 350°F. Bake for 35 to 45 minutes until the top starts to turn a deep golden color and the custard sets around the bread. At minute 35 check the middle with a thin knife and look for moist crumbs rather than liquid custard. You can also check the center with a food thermometer and aim for 165°F. If the top gets too dark before the middle sets, cover it loosely with foil for the last 10 minutes. Don't raise the oven heat to rush it. You'll notice crisp amber corners and a gentle wobble in the center when you move the dish. The kitchen will smell like warm vanilla and orange peel. (I found that a hotter oven made the top too dark before the center set.) Moderate heat lets the custard firm up without drying out.

Remove the baked pudding from the oven and let it stand for 15 minutes. Then use a sharp knife to cut it into 6 generous squares while the center is still warm. Slide a wide spatula under one serving and move it carefully onto a white dessert plate. The soft middle should hold together while some tender bread pieces stay slightly loose at the edges. Sprinkle 2 tablespoons powdered sugar over the six servings just before serving. Aim for a light dusting rather than a solid white layer. You'll see a creamy pale yellow inside beneath the crisp golden top. A few crumbs may fall away. That's fine. If you cut it straight from the oven, the soft center can collapse. Resting lets the custard settle so each portion holds its shape.