Clean a metal bowl with cotton wool to ensure it is totally clean and grease free.
Separate eggs, setting egg yolks aside for the custard.
Using an electric stand mixer or hand beaters, beat the egg whites on a medium setting until soft peaks form.
Once soft peaks appear, add sugar bit by bit, beating well between each addition to ensure the sugar is dissolved.
Turn the mixer up to high until the mixture thickens and the sugar has completely dissolved. It needs to reach the ribbon stage – when a spoonful of the meringue mixture poured back into the bowl sits on the surface like a ribbon.
Slow the machine, add the lemon juice, and sprinkle in the corn flour, then continue to beat until mixture is thick and glossy.
Prepare a baking tray by placing baking paper on it. Use a little of the meringue mixture to stick the paper onto the tray so it stays in one place while you are spooning the pavlova mix on to it.
Spoon the mix on to the prepared tray and mould it into the shape you want. Don't mess with the mix too much, though, as you want to get it in the quickly.
Place the tray in the middle of the oven and immediately turn the temperature down to 120 degrees Celsius. Don't use the fan.
Cook for 80-90 minutes, until the meringue has a hard crust, but don't let the pavlova colour. You may see a little sugar syrup seep out of the bottom but the crust should be firm. Inside, it will be soft and marshmallowy. Once cooked, turn oven off and leave pavlova inside with door ajar to cool slowly.
Whilst the pavlova is cooking, prepare the custard.
In a saucepan simmer the milk and lemon zest over medium heat. Just before it comes to the boil, turn off the heat and leave to infuse for five minutes. In a bowl cream the sugar and egg yolks together until they are fluffy and pale in colour.
Add the warm milk to the egg mixture, whisking to combine. Return to the saucepan and heat over a medium heat, stirring constantly to prevent it curdling. Do not let it boil.
Once it starts to thicken sprinkle in the cornflour and continue to stir until thick and able to hold its shape on top of the pavlova.
Take the custard off the stove and add the butter, bit by bit, whisking until well combined and glossy. Add the lemon juice.
Once the pavlova has cooled to room temperature remove from the oven and decorate with cooled citrus custard, seasonal fruit, nuts and any of your favourite toppings.