Preheat the oven to 140°C and line a tray with parchment.
Whisk three egg whites to soft peaks, then gradually whisk in 225g sugar, cornflour, and vinegar until glossy.
Bake spoonfuls of meringue for 10 minutes, then leave in the turned-off oven until crisp.
Purée 450g blackberries, custard, and lemon juice, then fold into whipped cream.
Whisk three egg whites with 225g sugar to stiff peaks and fold into the blackberry cream.
Spoon mixture into a lined 25cm springform tin and tap to release air.
Freeze the parfait for at least 4 hours or overnight until solid.
Heat 175g sugar and water in a pan until the syrup reaches 110°C.
Whisk three egg whites and lemon juice to firm peaks.
Remove sugar syrup from heat once it reaches 121°C.
Whisk syrup into egg whites in a steady stream for 5 minutes and place in a piping bag.
Fry butter and 200g blackberries, add liqueur, and flambé for one minute.
Assemble by plating crumbled meringue, parfait slices, piped Italian meringue, and warm blackberries.