Butter an 8x11-inch ceramic dish (I use an oval one, but any shape is good) with 1 teaspoon unsalted butter.
Arrange 13 ounces large panettone cubes or chunks in the prepared dish. See Notes below.
Lightly whisk 2 cups whole milk, 3 eggs, ⅓ cup light brown sugar, ½ teaspoon vanilla extract or paste and ½ teaspoon lemon zest in a large bowl. You don't need to beat it as we don't want to incorporate air, but make sure the eggs are well incorporated.
Pour the egg mixture carefully over the panettone, making sure all is moistened. Since the bread is usually uneven, some slices might be more covered than others, which is fine.
Let this mixture stand at room temperature, allowing the bread to absorb the liquid. If the panettone was still moist 20-30 minutes is fine. You might want to leave it longer if it is dry. You can also cover the dish with plastic wrap and refrigerate it for a few hours.
Preheat the oven to 375°F (190°C).
Check the mixture once or twice, submerging again any bread slices that popped to the surface too much. Or turn them over if they're on the surface and one of the sides is not being soaked enough.
Mix 1 tablespoon white sugar with ¼ teaspoon ground cinnamon and sprinkle on top.
Bake for 20 minutes, decrease the temperature to 325°F (160°C) and bake for another 20-25 minutes until the top is puffed and golden brown. A tester inserted should come out clean. Don't be tempted to leave it until it's very firm because we want it to be creamy. But also make sure the custard is not too runny.
Let cool a little and serve warm with whipped cream or vanilla ice cream. Drizzle some caramel or dulce de leche on top for an even more decadent dessert.
Store leftovers, covered, in the refrigerator.