Line a round cake tin with parchment and place it inside a larger baking dish.
Whisk egg yolks with yogurt until smooth.
Add 40 g sugar, cornstarch, baking powder and vanilla, whisking until lump‑free.
In a clean bowl, beat egg whites with salt, then gradually add 40 g sugar and whip to soft‑medium peaks.
Fold a spoonful of whites into the yogurt mixture to loosen it, then gently fold in the rest.
Pour the batter into the prepared tin and smooth the top.
Pour hot water into the outer baking dish to halfway up the sides of the cake tin.
Bake at 170°C / 340°F for about 50 minutes until set and lightly golden.
Cool in the tin, then remove, dust with icing sugar and top with berries or fruit if desired.