Pour milk into a small bowl, and add seeds scraped from vanilla bean. Whisk to combine. In a medium bowl, combine sugar, salt, cardamom, and flour. Add eggs and milk. Whisk until smooth and thick. Chill for at least 30 minutes.
Heat oil in a 4-quart saucepan over medium-high heat until a deep-frying thermometer registers 375 degrees F.
Attach rosette iron to handle. Carefully dip iron into oil for 2 to 3 minutes. Blot with paper towel. (It is very helpful to reheat rosette iron before dipping into batter.)
Dip hot iron into batter up to the top edge; do not allow batter to coat the top. Submerge iron in oil to bottom of pot for 15 seconds. Raise iron from bottom of pot, but keep submerged, and continue cooking until lightly browned. Remove iron from oil, and gently remove rosette, using a fork if necessary. If rosette pops off of the iron and falls into the oil, use tongs to retrieve it.
Drain rosettes on paper towels. Sprinkle with confectioners' sugar before serving.
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