Separate three egg yolks and egg whites into two large bowls.
Combine egg yolks, yogurt, salt, and lemon zest together and whisk well. Sift all-purpose flour and corn starch into the egg yolk mixture and continue whisking until there are no more lumps. Set aside.
Beat egg whites till frothy with an electric hand mixer. Add lemon juice and 1/3 of sugar while beating at high speed.
Slowly add the rest of the sugar to the egg white mixture while beating. Beat until medium peaks form.
Preheat the oven to 150°C (302°F).
Use a spatula and gently fold the egg white mixture into egg yolk batter in 3 batches until well combined. (Do not overmix.)
Pour batter into a 6-inch (15 cm) round springform pan lined with parchment paper. Tap the pan a few times down on the table to remove any air bubbles trapped in the batter.
Wrap the bottom of the cake tin with aluminium foil and transfer it to a bigger baking pan. Add boiling water to the bigger pan about 1 inch (2.5cm) high.
Bake for 30 minutes.
Then, change the oven temperature to 140°C (284°F) and continue baking for 30 minutes until browned.
Once the cake is done, leave the cake in the oven for 10 minutes before taking it out (with the oven off).
Remove from the oven. Let the cake cool down and remove it from the pan. Enjoy it immediately, or refrigerate for 3 hours before serving.