Prepare the Bread: Tear a 14 oz loaf of Challah bread into chunks and place them in a greased 9x13-inch baking dish.
Make the Custard: In a large bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, light brown sugar, vanilla extract, and cinnamon until smooth.
Soak the Bread: Pour the custard mixture evenly over the bread chunks, ensuring all pieces are coated. Cover and refrigerate overnight to let the flavors blend.
Prepare the Streusel Topping: When ready to bake, preheat your oven to 375°F. In a bowl, mix flour, brown sugar, and cinnamon. Cut in cold butter until the mixture becomes crumbly.
Bake the Casserole: Sprinkle the streusel topping over the soaked bread. Bake for about 50 minutes, until the top is golden and crispy, and the inside is soft and custardy.
Serve: Let it cool slightly before serving. Dust with powdered sugar and serve with fresh fruit or maple syrup for extra flavor.