Special equipment: an 8 by 8 by 2-inch baking dish
Preheat oven to 400 degrees F.
Butter baking dish and set aside. In a small bowl, combine butter, 1/4 cup sugar, cinnamon and salt. Generously butter both sides of brioche bread with butter mixture. Toast on a baking sheet in the oven 5 to 10 minutes, or until bread turns golden brown. Cut toast on a diagonal into triangles and arrange in the prepared baking dish. In a medium bowl, whisk together eggs, heavy cream, yogurt, orange juice, salt, remaining sugar, and vanilla. Stir in grapes and pour over bread slices in baking dish. Let rest 10 minutes for bread to absorb some of the egg mixture, gently pressing down a few times to help absorb. Bake 25 to 30 minutes until crust is golden and crispy in parts and custard is set. Remove from the oven, dust with powdered sugar, drizzle with maple syrup and serve.