½ cup (125ml) passionfruit pulp (see tip) OR lemon OR mandarin juice
100 grams butter
⅓ cup (75g) caster sugar
2 eggs, beaten lightly
2 egg yolks
Preheat oven to 160°C/325°F. Grease and line a 20cm (8in) round cake pan.
2 To make curd, combine your choice of fruit pulp/juice, butter and sugar in a medium heatproof bowl over a saucepan of simmering water over low heat, taking care that the bowl does not touch water. Stir until butter and sugar are melted. Gradually whisk in combined eggs and yolks; continue whisking over medium heat for 10 minutes or until mixture thickens. Remove curd from heat and pour into a clean small glass bowl. Refrigerate until chilled completely.
Beat butter, sugar and vanilla with an electric mixer until pale and fluffy. Add 1 egg at a time, beating well after each addition. Fold in sifted flour and buttermilk, alternating between them, until just combined.
Spoon two-thirds of the batter into pan; spread to smooth top. Spoon over the curd, leaving a 1cm (½in) border around edge of cake batter. Carefully spoon over remaining batter and spread to cover curd completely; smooth top.
Bake for 1 hour 20 minutes or until a skewer inserted into the centre comes out clean (some curd may come out on skewer when testing). Serve warm cake dusted with icing sugar.
If using passionfruit, you will need 6 for this recipe.