Preheat the oven to 350˚F. Grease a 9 x 13-inch oven-safe pan with 1 tablespoon of the butter and place inside a larger, high-sided roasting pan. Set aside.
In a small pot add the milk and lemon zest and stir to combine. Warm over medium-low heat for about 15 minutes to infuse, lowering the heat if necessary to ensure the milk does not boil. Remove from the heat and set aside to cool slightly.
Generously butter one side of each slice of panettone with the remaining butter. Arrange the slices in the prepared 9 x 13-inch dish in an even layer, stacking them as necessary.
Bring a small pot of water to a boil. Meanwhile, in a medium bowl, add the eggs and sugar and whisk until smooth and pale in color. Add the vanilla and orange flavored liqueur and whisk to combine. While whisking, slowly pour the warmed milk into the egg mixture in a slow and steady stream until completely combined. Spoon this mixture evenly over the panettone slices and sprinkle the brown sugar and almonds evenly over the top. Place the pan inside a larger, high-sided roasting pan and transfer to the middle rack of the preheated oven. Working very carefully, pour enough boiling water into the larger roasting pan so that it reaches about 1-inch up the side, being careful not to splash any of the water onto the bread pudding. Bake in the water bath until the pudding is set and golden brown on top, about 30-35 minutes. If the bread is getting too dark towards the end of the cooking time, cover loosely with aluminum foil to finish baking.
Carefully remove the pans from the oven. Lift the bread pudding pan from the larger roasting pan and set on a wire rack to cool for a few minutes before serving.
This pudding also works baked in square or oval dishes. It is just important you find a dish that fits all of the panettone slices and one that can go in the oven.