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1
Preheat the oven to 170C/150C fan/Gas 3.
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2
Place the egg yolks, sugar, water and oil in a large mixing bowl and beat together until smooth.
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3
Sift over the flour and salt and beat in along with the lime zest until well incorporated.
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4
In a separate bowl, whisk the egg whites to stiff peaks, then fold gently into the batter.
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5
Pour the batter into an ungreased 28cm, round, loose-bottomed tin and tap the tin lightly to expel any air bubbles.
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6
Bake for 6075 minutes, until well risen, golden and a skewer inserted into the centre comes out clean.
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7
Invert on to a wire rack to cool upside-down.
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8
Make the filling by whisking the cream and icing sugar to soft, floppy peaks, before folding in the mint, and rum if using.
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9
When the cake has cooled, remove from the tin and slice horizontally into two even layers.
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10
Place one on a cake board or large cake stand and spread it generously with the lime curd.
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11
Using a 10mm nozzle, fill a disposable piping bag with the cream and pipe in neat concentric circles over the curd in two layers, then top with the other cake half - I like to use the bottom half and place it upside-down so the flat bottom makes an evenly flat top.
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12
Make the topping by beating together the icing sugar and lime juice, using as much juice as is necessary to achieve a spreadable but not too runny water icing add more icing sugar or lime juice as necessary.
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13
Spread this on top of the cake and even out.
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14
Finish with a scattering of lime zest.
