A nice dish for a brunch buffet.
Make french toast: Butter 15 x 10 x 2-inch glass baking dish. Spread marmalade evenly over 1 side of each bread slice. Cut slices diagonally in half, forming triangles. Arrange triangles crosswise in dish, marmalade side up and overlapping slightly.
Whisk milk and next 7 ingredients in large bowl. Pour custard over bread. Let bread stand 1 hour, basting occasionally, or cover and refrigerate overnight.
Make citrus syrup:Mix first 5 ingredients in small bowl, stirring until sugar dissolves. Let stand at least 1 hour. (Can be made 1 day ahead. Cover and refrigerate.)
Preheat oven to 350°F. Bake French toast, uncovered, until puffed and golden brown, about 50 minutes. Sprinkle with powdered sugar; serve with citrus syrup.