Grease a large casserole dish with butter. Set aside.
In a large bowl, whisk eggs until light and fluffy. Add in pumpkin puree, brown sugar, milk, cream, cinnamon, nutmeg, salt, and vanilla extract. Pour a small amount (about 3/4 cup) into the casserole dish, then spread about 1/2 of the challah cubes into the pan. Sprinkle with 1/2 of the raisins. Pour half of the remaining egg mixture over the challah, then repeat process with remaining challah, raisins, and egg mixture. Cover tightly with plastic wrap and place in fridge overnight.
In the morning, preheat oven to 400°F. Remove french toast bake from fridge and remove plastic wrap. Bake, uncovered, for 25-30 minutes, until the challah is golden brown and crispy on top. Allow to cool slightly before serving with a dusting of powdered sugar and a large pour of maple syrup!