Adjust oven rack to middle position and preheat to 350°F (177°C). Line two half sheet pans with parchment and fit a large piping bag with a 1/2-inch round tip. Roll bag open and stand upright in a tall drinking glass so it can be filled hands-free. Have ready a small, fine-mesh sieve of powdered sugar for dusting. Fill a 2- or 3-quart saucier with a few inches of water; bring to a boil, then lower heat and adjust to maintain a steady supply of steam.
Combine eggs, sugar, and salt in the bowl of a stand mixer, using a flexible spatula to stir. Place over the steaming pot (if it touches the bottom, crumple a strip of foil into a ring to act as a booster seat) and cook, stirring and scraping constantly, until warmed to 160°F (70°C). This should not take significantly longer than 5 minutes; major delays simply indicate insufficient heat/lack of steam.
Transfer to a stand mixer fitted with a whisk attachment. Whip on high speed until the eggs are foamy, more than quadrupled in size, and thick enough to briefly mound up like soft serve when dropped from the whisk, from 5 to 10 minutes depending on the horsepower of your mixer. This is a crucial stage; if the foam is unable to hold soft peaks, the lady fingers will spread flat in the oven.
When eggs are foamy and thick enough to briefly hold their shape when dropped from the whisk, add lemon zest, if using, then sift the flour and cornstarch on top. Fold gently with a flexible spatula to combine in a thick batter. Transfer to prepared piping bag, twist to close, and pipe approximately thirty 3- by 1-inch fingers, leaving an inch between each one. When piping, hold the bag at a 45° angle and apply steady pressure as you pipe, then stop squeezing before you reach the end of each one, and lift the bag straight up to break the "tail" of batter.
Generously dust one tray with powdered sugar, and bake until puffed and firm to the touch, about 12 minutes. Set aside, then dust and bake the second tray as before. Cool ladyfingers to room temperature directly on the sheet pan, then transfer to an airtight container as soon as possible. If allowed to sit out after they have cooled, they will begin softening in the air. Use immediately, or store up to 3 weeks at room temperature.