Preheat the oven to 180C/350F/Gas 4 (fan 160C) and arrange an oven shelf in the bottom third of the oven. Sift the flour and 100g/3½oz of the caster sugar together in a bowl and set aside.
Whisk the egg whites in a large bowl with an electric hand whisk or mixer on a high speed for one minute until frothy. Add the lemon zest, lemon juice, cream of tartar and salt and continue whisking for 2-3 minutes, or until soft peaks form when the whisk is removed from the bowl. Increase the speed and add the remaining 200g/7oz of caster sugar, one tablespoon at a time to form firm, but not stiff peaks.
Sprinkle over one-third of the flour mixture and fold gently to combine. Repeat with the remaining two-thirds of the flour mixture folding gently to keep as much air in the mixture as possible.
Transfer the batter to a 25cm/10in angel food cake pan. Gently run a knife through the centre of the batter to remove any pockets of air. Cook for 45-50 minutes, or until a skewer inserted into the centre comes out clean.
Remove from the oven and immediately turn upside down onto the tin’s cooling legs, or place over the neck of a wine bottle. Leave to cool for at least one hour.
Run a knife around the inner and outer edges of cake to remove it from the pan. Invert onto a plate. Carefully use a palette knife to separate the cake from the base of the pan. Leave to cool on a wire rack.
For the lemon curd, mix the egg yolks, sugar, lemon juice and lemon zest together in a large pan. Cook over a low heat, stirring with a wooden spoon, making sure to stir the sides and base of the pan. Cook for 5-7 minutes, or until the mixture coats the back of a spoon. Remove from the heat and stir in the butter. Pass through a sieve into a large jug. Fill two 350g/12oz glass jars with the lemon curd and seal with lids. Cover the remaining curd with cling film and leave to cool.
For the topping, whisk the cream and vanilla extract in a bowl until soft peaks form when the whisk is removed. Spoon the topping over the angel food cake and, using a palette knife, coat the top and sides of the cake, smoothing as you go.
Cut the passion fruit in half and scoop out the seeds. Stir the passion fruit into the reserved, cooled lemon curd and drizzle over the angel food cake before serving. You may want to use just one of the jars of lemon curd to serve with the cake and save the other to eat separately.
Do not be tempted to grease the tin - it will prevent the cake from rising properly.
If you want to cut back on fat or have a dairy intolerance, this is a great cake to make. The cake itself doesn't contain any butter and you can easily swap the toppings for a fruit syrup or jam if you want to make it completely dairy-free.
The egg yolks are used up in a job lot of lemon curd, but you could always buy in a good jar lemon curd and save your yolks for another use.