In a medium bowl, whisk together the eggs, sugar, milk and vanilla until smooth. Add the flour and salt and whisk just until blended.
Heat an inch or two of oil in a heavy medium saucepan until it's hot but not smoking (if you have a thermometer it should register about 375°F). Dip the metal end of the rosette iron into the oil for a minute to heat up, then take it out, letting the extra oil drip into the pot.
Dip the rosette iron into the batter, without letting it come up over the top edge of the design, then submerge the iron in the oil and cook until golden, 35 to 40 seconds. Lift out iron, letting oil drip off, and pry the rosette off with a fork over a paper towel-lined plate. Repeat with remaining batter.
Dust the rosettes with icing sugar and serve immediately.
Serving Size Makes lots.