1.Make candied lime. Stir palm sugar, the water, juice and lemon grass in a medium heavy- based saucepan over low heat, without boiling, until sugar dissolves. Bring to the boil. Reduce heat, simmer, for 3 minutes or until syrup is reduced. Remove from heat, add lime slices, leave to cool in syrup. Drain lime slices, reserve slices and syrup separately. Discard lemon grass.
2.Preheat oven to 170°C (150°C fan forced). Grease a 26cm x 32cm (10½-inch x 12¾-inch) swiss roll pan, line base and long sides with baking paper, extending the paper 5cm (2 inches) over the sides.
3.Sift flours and soda into a medium bowl, stir in rind. Place sugar, butter and golden syrup in a small saucepan, stir over low heat until sugar dissolves. Stir warm butter mixture, egg and buttermilk into flour mixture. Pour mixture into pan.
4.Bake cake 12 minutes or until cake springs back when pressed lightly with a finger and shrinks away from side of pan slightly. Brush warm cake with 2 tablespoons of the reserved candied lime syrup. Cool in pan.
5.Line the base and sides of a deep 14cm x 23cm (5½-inch x 9¼-inch) loaf pan with baking paper.
6.Beat yoghurt and cream in a small bowl with an electric mixer until soft peaks form, fold in all but 1 tablespoon of the remaining reserved candied lime syrup and half the passionfruit pulp. Pour a little more than one-third of yoghurt mixture into loaf pan (this will be about 1½ cups of mixture). Trim a 12cm x 22cm (4¾-inch x 9-inch) piece from half the cake; carefully place in pan over yoghurt. Cover, freeze 1 hour or until firm. Refrigerate remaining yoghurt mixture. Cover remaining cake.
7.Pour remaining yoghurt mixture into pan. Trim a 14cm x 23cm (5½-inch x 9¼-inch) piece from remaining cake, carefully place on yoghurt layer. Cover pan, freeze 3 hours or overnight.
8.Turn cake onto a platter, stand 10 minutes before serving. Just before serving, top with lime slices. Combine remaining tablespoon of reserved candied lime syrup with remaining passionfruit pulp, drizzle over cake.
You will need to make this recipe the day before. You will need about 8 passionfruit for this recipe. You can substitute canned passionfruit pulp, if you like, however to compensate for the sugar added during canning add a teaspoon or two of lime juice. This recipe can be made up to 1 week ahead, cover tightly with foil, then freeze.