Preheat the oven to 180°C (350°F). Lightly grease a 23cm (9-inch) springform pan and line the sides with parchment paper.
Separate the egg whites from the yolks. Place the yolks in a large mixing bowl and set aside. Beat the egg whites in another bowl until you get stiff peaks, 4-5 minutes on high speed with an electric mixer.
4 large eggs
To the egg yolks, add the strained yogurt, sugar, lemon zest, lemon juice, and vanilla. Beat (you can use the same beaters as for the whites) until well combined, about a minute.
400 grams (1 ¾ cups) strained yogurt, 80 grams (⅓ cup) sugar, Zest of a lemon, Juice of a lemon, 1 teaspoon vanilla extract
Add the flour, arrowroot or cornstarch, baking powder, and salt to the egg yolk mixture. Beat on low speed until combined.
50 grams (⅓ cup) light spelt flour, 1 tablespoon arrowroot flour, ½ teaspoon baking powder, ½ teaspoon fine sea salt
Add the beaten egg whites to the mixture and gently fold in with a wooden spoon or spatula until well incorporated. The mixture should be very fluffy looking with no lumps of egg white.
Transfer the batter to the prepared springform pan. Bake for 45-50 minutes. The top will be browned and the cake might crack. It will be rather tall when you take it out of the oven, and then fall to about half that height.
Cool for 10 minutes before carefully removing the sides of the pan. Cool fully on a wire rack before serving with any toppings. If you're not sure about removing the base of the pan, simply leave it and slice and serve without taking it off.
Leftovers can be kept in a sealed container at (cool) room temperature or in the refrigerator for a couple of days. It doesn't freeze well.
Notes
To get 400 grams of strained yogurt, you will need to start with 800 grams (two standard containers) of Greek-style thick coconut or soya yogurt. Place into a tea towel or layered cheesecloth with a pinch of salt and a squeeze of lemon juice, then tie tightly. Hang from a wooden spoon or place in a sieve over a tall or large container of some kind - quite a lot of water will drip out - and refrigerate overnight. In the morning you should have a very thick yogurt (see in the ingredient image). Read more on this process for vegan labneh.You can bake this in a 20cm (8-inch) springform, adding about five minutes to the baking time. I don't recommend baking it in a larger tin or a pan that doesn't have removable sides.If you don't like the taste of the yogurt you have chosen, you won't like the cake. If you don't like coconut, don't use coconut yogurt (use soya). There are many options for Greek-style dairy-free yogurt these days, and some of them are excellent, so choose a good one!