Preheat oven to 350°F (175°C) and butter an 8x8-inch baking dish. Cut panettone into 1-inch cubes, spread on a baking sheet, and toast 5-7 minutes until dry and lightly golden.

Whisk eggs, sugar, milk, cream, vanilla, cinnamon, and salt in a large bowl until smooth.

Add toasted panettone cubes to the custard and stir to coat. Let soak 15 minutes, stirring once.

Transfer the mixture to the buttered dish, press down lightly, and bake 35-40 minutes until puffed, golden, and the center barely jiggles.

Let rest 5 minutes, then scoop into bowls and serve warm with powdered sugar or whipped cream.