1. Cut brioche bread into 1/2" to 1" chunks. Preheat the oven to 350ºF with a rack in the middle. Grease a 2 quart casserole pan with butter or cooking spray.
2. In a large bowl whisk together eggs, milk, vanilla, cardamom, baking soda, and salt. Add in bread and gently stir to coat bread. Transfer mixture to the prepared baking dish and set aside.
3. Make the cardamom maple syrup. Heat the butter until just melted. Stir in the maple syrup, cardamom, and orange zest. Drizzle the syrup over the french toast, coating as evenly as possible.