1.Make lime curd. Whisk eggs, sugar and lime juice in a medium saucepan continuously over low heat for 3 minutes or until thickened to the consistency of custard. Whisk in butter, a small piece at a time, until smooth. Do not boil. Cover surface with plastic wrap; refrigerate for 6 hours or overnight. Stir in rind.
2.Preheat oven to 180°C/350°F. Grease a 20cm (8-inch) round cake pan; line base and side with baking paper.
3.Beat butter, caster sugar and rind in a medium bowl with an electric mixer until pale and fluffy. Beat in eggs, one at a time, until combined. Fold in chocolate, then flour, nuts, coconut and buttermilk. Spoon mixture into pan; smooth surface.
4.Bake cake for 50 minutes or until a skewer inserted in the centre comes out clean. Stand cake in pan for 5 minutes before turning, top-side up, onto a wire rack to cool.
5.Beat cream and icing sugar in a small bowl with an electric mixer until soft peaks form.
6.Level the top of the cake if necessary; split cake into two layers. Place the bottom cake layer on a plate; spread with half the lime curd and half the whipped cream. Finish with top cake layer, remaining lime curd and cream. Decorate with mango slices and finger limes.
You will need to start this recipe the day before. Finger limes are an aromatic native Australian citrus available from selected greengrocers from March to June. Cut in half lengthways to scrape out the juice sacs (vesicles). The cake, curd, syrup and rind can all be made a day ahead. Store the cake in an airtight container at room temperature; refrigerate curd, syrup and rind separately. Assemble close to serving.