Cut butter into small cubes and add to the mixing bowl and melt for 1 minutes / 150°F|65°C / speed 1.
Add eggs, milk, sugar, vanilla, cinnamon, and salt to the mixing bowl and mix for 10 seconds / speed 5.
Insert the butterfly whisk.
Cut or tear challah or brioche into 1-inch (2.5cm) pieces and add to the mixing bowl along with blueberries and blackberries and mix for 10 seconds / speed 1 / stir.
Line the deep steamer basket with 3 layers of foil so the foil extends over the sides.
Crimp the foil so it fits the steamer snugly and spray the foil with cooking spray.
Use spatula to scrape French toast mixture into the steamer.
Use spatula to press down bread mixture so the bread soaks up the milk-egg mixture.
Wash mixing bowl.
Pour 4 cups water into mixing bowl.
Attach deep steamer basket to mixing bowl.
Place lid on steamer basket and steam for 40 minutes / 265°F|130°C.
Remove steamer basket.
Dust French toast casserole with powdered sugar and serve warm with maple syrup.
Enjoy your meal!