Grease only the bottom of a 9-inch round cake pan and cover with a circle of parchment paper. Do not grease the sides of the pan.
In a large bowl, beat egg yolks until light, gradually adding in the sugar. Add in vanilla and lemon and set aside.
In a medium bowl, beat the egg whites until frothy, about 1 minute at high speed, then beat in the salt and cream of tartar. Continue beating until the eggs reach soft peaks. Fold egg whites, working in 3 or 4 batches, into the yolk mixture.
Sift the flour over the egg mixture, again working in 3 or 4 batches, and gently fold it inches When the batter is uniform, pour into prepared cake pan and bake unil a tester comes out dry and the top of the cake springs back when gently pressed, about 35-45 minutes.
Allow cake to cool in the pan before cutting around the sides with a sharp knife and turning it out.
Slice the cake into 2 or 3 layers (or squares) and fill as desired.