Preheat the oven to 180°C. Separate yolks and egg whites. Whip the yolks and the sugar together with the lemon juice and the scraped vanilla pod seeds using an electric mixer until they are light, white and creamy. Add the flour and the lemon zest and mix until thoroughly incorporated.
In another bowl whip the egg whites until stiff peaks form. Gently fold in the whipped egg whites into the mixture. The final mixture should be light, white and very soft.
You need to bake now 6 circle sponge cakes: you can use a 25 cm diameter mould, previously greased and dusted with flour, dividing the batter into 6 parts and baking one cake after the other, or you can spread one sixth of the dough on a rectangular tin lined with parchment paper and then cut out a circle with a large mould.
Whatever method you choose, pour the batter onto the baking sheet and bake the 6 circles for 5-7 minutes until golden brown. As soon as it is ready, remove the cake from the oven, unmould or remove the parchment paper and place on a wire rack to cool. Set them aside, separated by parchment paper, and let them cool.