Step 1
Arrange the panettone bread cubes to fit compactly into a buttered 2 quart casserole dish. Do not compress too tightly; set aside any leftover bread cubes.
Step 2
Whisk together 3 eggs and 1/2 cup sugar until the sugar is dissolved and the mixture becomes light yellow in color. Pour in the cream vanilla and 1 tablespoon orange liqueur; whisk to combine. Stir in 1 pinch salt 1/8 teaspoon nutmeg lemon zest and orange zest. Pour this mixture over the panettone cubes in the casserole dish. Cover and refrigerate for 30 minutes.
Step 3
Preheat oven to 350 degrees F (175 degrees C).
Step 4
Remove the bread from the refrigerator uncover and dot the top of the bread pudding with 2 1/2 tablespoons butter and sprinkle with 2 tablespoons of white sugar.
Step 5
Set casserole dish into a shallow pan that is 2 to 3 inches larger in diameter than the casserole dish. Set these pans onto the center rack of the preheated oven. Pour hot water into the outside pan until it reaches a depth of 1 inch around the casserole dish creating a water bath.
Step 6
Bake the pudding in the water bath until it is fully set and a knife inserted into the center of the pudding comes out clean 1 to 1 1/4 hours. The water bath will help the pudding to cook evenly and maintain a light texture. Carefully remove the pudding from the water bath and cool for at least 15 minutes before serving.
Step 7
Just before serving make the sauce by melting 1/2 cup butter in a saucepan over low heat. Stir in 1 cup sugar 1/4 cup orange liqueur 3 tablespoons water 1/8 teaspoon nutmeg and 1/8 teaspoon salt. Stir and heat over medium heat until the sugar is fully dissolved and the liquid is heated through. Remove from heat.
Step 8
In a small mixing bowl whisk 1 egg until well beaten. While whisking the egg slowly pour 2 tablespoons of the hot mixture into the bowl with the egg. Then while whisking mixture vigorously slowly pour the warmed egg mixture back into the sauce.
Step 9
Place the saucepan back over low heat gently stirring the sauce and raising the temperature slowly to medium. Continue to stir until the sauce almost reaches a simmer reducing the heat as necessary if the mixture threatens to boil. Continue stirring until the sauce thickens 1 to 2 minutes. Spoon over the bread pudding and serve immediately.