Preheat the oven to 180C. Grease and flour a 26cm-diameter fluted ring tin.
Whisk the eggwhites with a pinch of salt until soft peaks form
Beat the butter with the sugar using the paddle attachment of an electric mixer. Add the egg yolks one at a time, followed by the lemon rind, then the yoghurt. With the mixer on the slowest speed, gradually add the flour and baking powder. Fold in the eggwhites.
Spoon the mixture into the cake tin and bake until you can tell it is fully cooked by inserting a knife in the middle – you’ll know it’s ready when the knife comes out clean (40-45 minutes).
Once out of the oven and while still hot, turn out and dust the cake with icing sugar.