Soak Bread: Place the bread cubes and milk in a big bowl, making sure the bread is submerged in the milk for the most part. For at least one hour and up to overnight, cover and refrigerate.
If you store eggs in the refrigerator, remove them after an hour or so to allow them to return to room temperature.
After the bread has soaked, heat the oven to 375 degrees Fahrenheit.
To assemble, melt the butter in a small bowl, then lightly spray the baking dish with a tiny amount of the melted butter and set it aside.
Whisk together the eggs, sugar, vanilla, melted butter, and chai spice in a medium-sized bowl. Pour part of the milk gently into the dish containing the eggs and sugar from the big bowl containing the bread. Whisk gently. Transfer the mixture to the spacious basin containing the bread, and use a spatula to gently toss to evenly distribute the sugar-chai mixture over the bread.
Transfer to the baking dish.
Bake: Place the baking dish in the oven and bake it for 45 minutes with the foil covering it. When everything has set and a tester or a sharp knife pulls out clean from the middle, remove the foil and continue baking for an additional 20 minutes.
Serve: Add some powdered sugar on top, then serve!