Note: If day-old bread isn’t an option, cut the cubes and place on two sheet pans. Place in a 300 degree F. oven until slightly dry but not toasted. Bread should be still a little soft when squeezed.
Spray or grease a 3 quart baking dish. Place the bread in casserole. In a large measuring cup or bowl, combine milk, cream, and vanilla; drizzle over bread.
In same measuring cup or bowl, whisk together eggs and sugar. Whisk in flour, baking powder, and salt. Whisk in melted butter. Drizzle over bread in baking dish. Gently press down on bread cubes with the back of a spoon or spatula to submerge bread in batter mixture. Cover and chill 1 hour or up to overnight.
Preheat oven to 350°. Bake, uncovered, until evenly puffed and golden brown on top, 40 to 45 minutes. (A knife inserted in the center should come out clean, or an instant-read thermometer inserted in the center should register 165°.) If browning too fast, cover lightly with foil.
Dust with powdered sugar and serve with syrup if desired.