Heat the oven to 120 C / 250 F with a rack in the center. Draw a 23 cm / 9-inch circle in pencil on parchment, flip it pencil-side down, and set it on a 33 x 46 cm / 13 x 18 inch baking sheet. Separate the eggs, putting 6 whites in the mixer bowl and keeping 3 yolks in the fridge for the curd. Wipe the bowl and whisk with a paper towel dampened with lemon juice.

Whisk the whites on medium speed for 2 to 3 minutes until foamy and holding soft peaks. With the mixer running, add the caster sugar 1 tablespoon at a time, pausing 10 to 15 seconds between additions.

Raise the speed to high and whip for 8 to 10 minutes until the meringue is stiff, glossy, and holds a peak when you lift the whisk. Rub a little between your fingers. If it feels gritty, the sugar hasn't dissolved, so keep whipping 2 to 3 minutes more or the shell will weep syrup as it bakes. Add the cornflour, lemon juice, and vanilla, and fold with a spatula until just combined.

Spoon the meringue onto the circle and spread it to the edge, building the sides slightly higher than the center to leave a shallow well. Drag the back of a spoon up the sides to make vertical ridges.

Bake for 90 minutes without opening the door, until the shell is dry and firm to the touch and pale ivory with no browning. Turn off the oven and leave the pavlova inside with the door closed for 60 minutes to cool.

While the shell bakes, whisk the 3 yolks, granulated sugar, lemon zest, lemon juice, and salt in a 2-quart / 1.9 L heavy saucepan. Cook over low heat, whisking constantly, for 8 to 10 minutes until the curd coats the back of a spoon and reads 77 C / 170 F on an instant-read thermometer.

Take the pan off the heat and whisk in the butter a few cubes at a time until glossy. Strain through a fine-mesh sieve into a bowl, press plastic wrap onto the surface, and refrigerate for at least 2 hours. That fits inside the bake and cooling window.

Peel the parchment away from the cooled shell and set the shell on a serving platter. Whip the cold cream with the powdered sugar and lemon zest to soft peaks, about 2 minutes, then fold in 4 tablespoons of the chilled curd for a loose ripple.

Pile the cream into the center of the shell, drizzle with the remaining curd, and scatter the berries and mint on top. Serve within 30 minutes, cutting with a serrated knife.