Preheat the oven to 350 degrees. Prepare a round 8-inch cake pan by buttering the bottom and lining it with parchment or wax paper.
Using a citrus zester, zest the lemon and set aside
Add the eggs to a large mixing bowl. Using an electric mixer mix on medium speed to break up the eggs. If you have a whip attachment, that works the best, but the regular beaters will also work.
Gradually add the sugar to the eggs and beat for about 10 minutes on medium-high to high speed until the egg mixture has tripled its volume. When properly beaten, the mixture should fall from the whipping attachment in ribbons and slowly dissolve in 10 seconds. The batter will look thick almost like cake batter.
Beat in the vanilla and lemon zest.
Sift ¼ cup of flour and salt on top of the batter and slowly fold it into the batter with a rubber spatula. Add the remaining flour in ¼ cup increments, folding until no flour appears. The batter should still be foamy. If you over-stir, the batter will deflate.
Pour the batter into the prepared pan, and with a toothpick or skewer, drag it through the batter in a spiral to remove the air bubbles, and then gently tap the cake pan on the counter.
Bake the cake for 25-30 minutes then let cool for 10 minutes. You will see the cake shrink away from the pan, take a serrated knife and go around the edge of the pan to loosen it, then remove the cake and turn it upside down on a plate or platter. Remove the parchment paper and let the cake completely cool.