Preheat oven to 190°C. Using an electric mixer, beat egg yolks and 1/2 the caster sugar until very thick and pale. Combine oil, lime rind and 3/4 cup water in a jug. With mixer on low speed, gradually pour mixture in, beating until just combined.

Using an electric mixer, beat eggwhites until foamy. Add cream of tartar and beat until soft peaks form. Gradually add remaining caster sugar, 1 tbs at a time, continuing to beat until thick and glossy.

Sift flour, coconut and salt over yolk mixture and whisk until just combined. Using a large metal spoon, fold in eggwhite mixture in 2 batches. Spoon mixture into an ungreased 25cm tube (angel food) cake tin (from kitchenware stores). Level top then bake on lowest shelf in oven for 45-50 minutes or until a skewer inserted into the centre comes out clean. Immediately invert cake tin, supporting it on its legs. Cool, inverted in tin (cooling the cake suspended prevents it from sinking).

Using a palette knife, run knife around inner edge of cooled cake to release it from tin. Turn on to a plate. Combine icing sugar and enough lime juice to form a thick icing then spoon over cake, allowing it to drip down sides. Stand until set. Meanwhile, for caramel berries, combine sugar and 1/4 cup water in a small saucepan. Stir over low heat until sugar dissolves, then simmer without stirring until mixture forms a dark caramel. Remove from heat. Add lime juice and 1/4 cup water and stir well. Top cake with strawberries and raspberries, then place remaining berries in a heatproof bowl. Pour hot caramel mixture over and cool. Drizzle a little of the mixture over top of cake and serve with remaining berries.