Heat oven to 350℉/177℃.
Line sheet pan with parchment.
Spray a 9-inch x 13-inch casserole with cooking spray. Set aside.
Place cubed panettone on prepared sheet pan.
Bake until just starting to brown.
Set aside to cool.
Melt butter. Set aside.
In a large bowl, whisk together milk, cream, eggs, sugar, salt, vanilla and melted butter.
Place panettone in prepared casserole.
Pour milk mixture over top of panettone.
Let stand for 30 minutes, making sure to press bread down into milk mixture.
Bake for 90 minutes, or until puffed and set. (You may need to cover it with foil after 45 minutes.)