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. Cut 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 eggs, yolks, sugar, half and half, rum, vanilla extract, and a healthy grating of fresh nutmeg. Pour this over the bread cubes.
Spray a 12 hole 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 tall sided cookie sheet or roasting pan. Transfer to the preheated oven and carefully pour the hot water from kettle onto sheet pan, creating a water bath for the muffin tin to sit inches.
Bake 15 to 20 minutes until the tops are nicely browned, and a toothpick comes clean from the center.
Bread pudding may be served warm or cold. Vanilla Ice cream or whipped cream would be great accompaniments.
Alternately 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 to 20 minutes. The puddings will puff and a toothpick will come out clean from the center. The tops won't brown with this method. Perhaps a sprinkle of cinnamon sugar at serving time would be nice.