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Lightly grease a 9x13 in baking dish and layer the challah pieces into the dish.
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In a large bowl, whisk together the eggs, half and half, sugar, vanilla, and cinnamon. Pour the egg mixture over the bread, cover the baking dish with plastic wrap, and let rest in the fridge for at least 2 hours, or up to 1 day.
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Prepare the topping by combining brown sugar, flour, cinnamon and salt in a bowl and whisking to distribute ingredients well. Cut butter into small cubes and add to dry ingredients. Mash in with a fork or pastry cutter. Stir in chopped pecans. Cover and place in fridge.
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When ready to bake, preheat the oven to 375 and remove the dish from the fridge. Sprinkle the topping all over the casserole. Bake until puffed and deep golden brown, 35 to 45 minutes.