Preheat your oven to 170˚C (340˚F). Grease a 23 cm (9-inch) round baking pan and line the bottom with wax paper.
In a mixing bowl, put the whole eggs and the sugar. Beat on medium-high speed for approximately 10 to 12 minutes, or until the mixture has increased in volume and appears very fluffy and light in color. When lifting the wire whisk, the cream should be consistent and firm, not runny.
Add the vanilla and continue beating for one more minute.
In a separate bowl, mix the flour, salt, and baking powder. Add cinnamon if desired.
Sift the dry ones over the egg mixture and with a rubber spatula mix with enveloping movements, never quickly or frantically because we would lose all the air that we incorporate during the beating.
At this point you can add the lemon zest if you want to use it. Mix gently from the bottom of the bowl to the surface, as lumps of flour tend to settle at the bottom.
Put the mixture in the mold and bake for approximately 25–35 minutes (depends on your oven, the shape of your mold and the material of your mold). Do the toothpick test in the center of the mold to verify that the cake is cooked, if the toothpick comes out wet it is because it is still raw. If it comes out dry, it is ready for you to remove from the oven.
After 5–8 minutes at room temperature, remove from the mold and place on a rack to allow the cake to cool completely and the air to flows all around the cake.