John says: "This cake is absolutely perfect for long sunny evenings, and it goes without saying that it should be accompanied by a tall glass of the liquid form. The rum part is entirely optional but let's face it: without the rum there's no mojito!"
1. Make the lime curd: place the lime juice, butter and zest into a medium saucepan and heat over medium/high.
2. Meanwhile whisk together the yolks and caster sugar - adding a tablespoon of lime juice if it’s a little thick - then whisk in the cornflour.
3. When the butter melts into the lime juice and it comes to the boil, pour half onto the eggs and sugar, whisking constantly to blend, then pour that back into the pan and heat again, whisking constantly until the mixture thickens and coats the back of a spoon - though when testing for this make sure you take the pan off the heat!
4. Scoop it onto a plate and cover with cling to chill, or into a sterilised jar with lid.
5. For the cake, preheat the oven to 170C/150C fan/Gas 3
6. Place the egg yolks, sugar, water and oil in a large mixing bowl and beat together until smooth. Sift over the flour and salt and beat in along with the lime zest until well incorporated.
7. In a separate bowl, whisk the egg whites to stiff peaks, then fold gently into the batter. Pour the batter into the tin and tap the tin lightly to expel any air bubbles. Bake for 60-75 minutes, until well risen, golden, and a skewer inserted into the centre comes out clean. Invert on to a wire rack to cool upside down.
8. Make the filling by whisking the cream and icing sugar to soft, floppy peaks, before folding in the mint and rum if using.
9. When the cake has cooled, remove from the tin and slice horizontally into two even layers. Place one on a cake board or large cake stand and spread it generously with the lime curd. Fill the piping bag with the cream and pip 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.
10. Make the topping by beating together the icing sugar and lime juice, using as much juice as is necessary to achieve a speadable but not too runny water icing - add more icing sugar or lime juice as necessary. Spread this on top of the cake and even out. Finish with a scattering of lime zest.
For the lime curd:
250ml fresh lime juice
125g unsalted butter, cut into cubes
Zest of 1 lime
3 egg yolks
175g caster sugar
For the cake:
9 eggs, separated
450g golden caster sugar
160ml sunflower oil
380g plain flour
3 limes, zest only
For the filling:
400ml double cream
200g icing sugar
10 large mint leaves, finely chopped
3tbsp white rum (optional)
1 quantity lime curd, as above
For the topping:
250g icing sugar
Juice of 1/2 - 1 lime
Lime zest to decorate
28cm/11" round, loose-bottomed cake tin, ungreased
Disposable piping bag fitted with 10mm nozzle