Whisk first 6 ingredients together in a medium bowl. Soak each bread slice in the egg mixture and arrange in 13x9x2-inch baking dish. Pour remaining egg mixture evenly over bread. Allow to stand until egg mixture is absorbed (at least 20 minutes.) (May be prepared 1 day ahead.) Cover and refrigerate.
Melt 2 tablespoons butter in large heavy skillet over medium heat. Add 4 bread slices to skillet and sauté until cooked through and golden brown, approximately 3 minutes on each side. Repeat cooking with remaining 2 tablespoons butter and 4 bread slices.
Sift powdered sugar over each slice. Serve with maple syrup and garnish with a twisted orange slice