Start by preheating the oven to 350°F (180°C).
Using a mixer, high speed, whip butter, sugar and lemon zest for about 15 minutes, until you get a very light, snowy cream.
In another bowl, add whole eggs, egg yolks and vanilla paste, beating the mixture lightly with a fork.
Add slowly (using medium speed) the egg mixture. Add ⅕ of it, then add more just after the eggs are fully incorporated. The eggs must be at room temperature to prevent the batter from curdling.
Batter will be fluffy like the previous creamed mixture, but more yellowish because of the egg yolks.
In another bowl, mix the dry ingredients: starch, flour, salt and baking powder.
Start adding the dry mixture in small portions, mixing at low speed and only adding more when the dry ingredients are fully incorporated into the batter.
Butter or line with parchment paper a round baking pan.
Pour the batter, distributing it evenly. Use a spoon or spatula to even the surface.
Bake for 40 minutes. When cake is ready, the top should be brown, and a toothpick must come out clean when inserted in the middle.
After 15 minutes, remove from baking pan, placing it in a cool rack for cake to cool. Top it with icing sugar.
Wait at least 40 minutes for it to cool before serving.