Tippaleipä (Finnish May Day Funnel Cakes)

Heat enough oil in a 6-qt. saucepan to reach a depth of 2" until a deep-fry thermometer reads 375°. Whisk sugar, salt, eggs, and zest in a bowl until smooth. Alternate whisking in milk and flour until a smooth batter forms. Transfer batter to a piping bag fitted with a plain 1⁄4" tip. Working in a swirling motion, pipe batter into oil until an 8" swirled cake forms. Fry, flipping once, until golden and slightly crisp, 2-3 minutes. Using a slotted spoon, transfer cake to paper towels to drain; repeat with remaining batter. Let cakes cool slightly; dust with confectioner's sugar before serving.