Cloud Cake (Fluffy Greek Yogurt Cake, No Flour)

Heat the oven to 320°F (160°C). Line the bottom of an 8-inch (20 cm) springform pan with parchment and wrap the outside tightly with two crossed sheets of heavy-duty foil. Put a kettle of water on to boil.
In a large bowl, whisk the Greek yogurt, egg yolks, 1/4 cup (50 g) of the sugar, the cornstarch, lemon zest, lemon juice and vanilla until smooth.
In a clean bowl, beat the egg whites with the salt on medium speed until foamy. Add the remaining 1/4 cup (50 g) sugar a spoonful at a time and beat until you have firm, glossy peaks that only bend at the tip.
Stir one third of the whites into the yogurt mixture to loosen it, then gently fold in the rest in two additions until no big lumps of white remain.
Pour the batter into the pan and smooth the top. Set it in a roasting pan and pour in hot water about 1 inch (2.5 cm) up the sides of the springform.
Bake 50 to 55 minutes, until puffed, pale golden and set with a slight jiggle in the center (160°F / 71°C on an instant-read thermometer).
Turn the oven off, crack the door open a few inches and leave the cake inside for 30 minutes. Lift it out of the water bath and cool to room temperature.
Cover and chill at least 3 hours, ideally overnight. Release the pan, dust with powdered sugar and slice with a hot, wiped knife.