Pour milk into a small bowl and add the seeds scraped from the vanilla bean. Whisk to combine. In a bowl, combine the sugar, salt, cardamom and flour. Add the eggs, alternating with the milk. Whisk until smooth and thick. Let the batter stand for at least 30 minutes or refrigerate overnight.
Heat the oil in a 4-quart saucepan over medium high heat until a deep frying thermometer registers 375 degrees F.
Attach the rosette iron to its handle. Carefully dip the iron into the oil for 2 to 3 minutes. Blot with paper towel. Always reheat the rosette iron before dipping into the batter.
Dip the hot iron into the batter up to the top edge. Do not allow the batter to coat the top of the iron. Submerge the iron in the hot oil and cook the rosette until lightly browned. Remove from the oil and gently remove the rosette from the iron, using a fork if necessary. If rosette pops out of the iron and falls into the oil, use tongs to retrieve it.
Drain rosettes on paper towels. Sprinkle the rosettes with the confectioners' sugar before serving.