Preheat the oven to 400°. Butter a 2-quart baking dish. Toast the panettone on the oven racks for 8 to 10 minutes, or until golden; cut into 1 1/2-inch cubes and transfer to the prepared baking dish.
In a saucepan, cook the butter over moderately low heat until nutty and the milk solids are brown, 4 to 5 minutes. Toss the butter with the panettone.
In a medium bowl, using an electric mixer, beat the eggs and sugar at medium speed until pale and thick, 2 minutes. Beat in the vanilla, cinnamon, nutmeg and salt, then beat in the heavy cream and evaporated milk. Pour the custard over the panettone; let stand for 1 hour, gently tossing now and then, until the custard is absorbed.
Preheat the oven to 400°. Bake the bread pudding for 25 to 30 minutes, until the top is golden and the custard is set. Let cool for 30 minutes before cutting into squares.
The pudding can be refrigerated overnight. Rewarm before serving.