Pre-heat the oven to 475°F.
Place a tea kettle of water on to boil for a water bath. With a serrated knife, remove the side crusts from the half piece of the panettone. Cube into 3/4 to 1-inch dice. You should have 5 cups. Reserve the cubed panettone in a large mixing bowl.
For the eggnog custard, in another bowl, thoroughly whisk together the three whole eggs, three egg yolks, sugar, half-and-half, rum, vanilla extract and a healthy grate of fresh nutmeg. Pour the mixture over the bread cubes.
Spray a 12-cup muffin tin with vegetable cooking spray. Ladle the bread/eggnog mixture gently and evenly into the muffin tins. The big cubes sticking up look nice. Place the filled muffin tin in a rimmed cookie sheet or roasting pan. Transfer to the pre-heated oven and carefully pour the hot water from kettle onto the sheet pan, creating a water bath for the muffin tin to sit in.
Bake for 15-20 minutes, until the tops are nicely browned and a toothpick comes clean from the center.
Alternatively, place the muffin tin in a roasting pan on the stove. Pour in up to 1 inch of boiling water. Cover and cook over medium-high heat for 15-20 minutes. The puddings will puff and a toothpick will come out clean from the center. The tops won’t brown with this method. You may add a sprinkle of cinnamon sugar at serving time.
Bread pudding may be served warm or cold. Vanilla ice cream or whipped cream work well as accompaniments.