For the Chai Spice Mixture:
1 1/2 teaspoons ground cinnamon
1 teaspoon ground cardamom
1 teaspoon ground allspice
1/2 teaspoon ground ginger
1/8 teaspoon ground cloves
For the French Toast:
4 large eggs
1 1/2 cup 2% or whole milk
1/3 to 1/2 cup granulated sugar (use less if serving with maple syrup)
1/2 tablespoon pure vanilla extract
1 tablespoon plus 1 teaspoon Chai Spice Mixture
1 tablespoon unsalted butter, softened
9 to 10 cups day-old challah bread, chopped into cubes
For the Topping:
1/4 cup all purpose flour
1/4 cup packed brown sugar
1/2 teaspoon Chai Spice Mixture
3 tablespoons unsalted butter, cold and chopped into small pieces
In a small bowl, whisk together the cinnamon, ginger, cardamom, allspice and cloves for the Chai Spice Mixture. Set aside.
For the french toast, whisk together the eggs, milk, sugar, vanilla extract and 1 tablespoon plus 1 teaspoon of the Chai Spice Mixture in a medium bowl.
Grease a 9x5-inch loaf pan with the butter and layer the cubed challah inside the pan, gently pressing it down. Briefly re-whisk the liquid ingredients and pour them evenly over the bread, taking care to cover all of the exposed bread. Cover with plastic wrap and refrigerate for a minimum of 30 minutes or as long as overnight to allow the mixture to absorb into the bread.
When ready to bake, preheat the oven to 350°F. In a small bowl, combine the topping ingredients until crumbly.
Uncover the loaf pan and gently press the bread down evenly until the liquid begins seeping up. Spread the topping evenly over the bread. Bake for 45 minutes until the top is golden and crunchy.
Allow to cool for several minutes and then slice and serve.