Preheat oven to 180°C and sift flour with 100g sugar.
Whisk egg whites until frothy, then add zest, juice, cream of tartar, and salt to form soft peaks.
Gradually whisk in 200g sugar until firm peaks form.
Gently fold the flour mixture into the egg whites in three batches.
Transfer batter to a 25cm pan and bake for 45-50 minutes until a skewer comes out clean.
Invert the pan immediately and cool for at least one hour before removing the cake.
For curd, cook yolks, sugar, juice, and zest over low heat until the mixture thickens.
Remove curd from heat, stir in butter, strain, and jar most of it while reserving some for the cake.
Whisk cream and vanilla to soft peaks and coat the top and sides of the cooled cake.
Mix passion fruit seeds into cooled curd and drizzle over the cake.