Preheat the oven to 140C/gas 1/fan 120C and line a baking sheet with non-stick parchment paper. In a small bowl, blend the cornflour, vanilla extract and vinegar to a smooth paste.
Whisk the egg whites until stiff in a separate clean bowl. Gradually whisk in the sugar a little at a time to make a thick and glossy meringue, then whisk in the cornflour paste until well combined.Spoon the mixture on to the paper, then spread, without flattening it, to form a 23cm/9in round. Swirl the edges with the back of the spoon to give lovely soft folds and peaks. Scatter half the pistachios over the meringue and bake for 1 hour, by which time it will feel crisp if you tap it. Turn off the oven and leave the pavlova to cool with the oven door open.
Set aside 450g/1lb of the apricots and roughly chop the rest, discarding the stones. Purée the chopped apricots, then push them through a sieve with a metal spoon. Stir in the liqueur and sweeten with 3 tbsp of the icing sugar, or enough to sweeten as you like it.
Lightly whip the cream so it’s soft and billowy, then spoon over the pavlova. Cut the reserved 450g/1lb apricots in half, discard the stones, then cut into wedges. Scatter them over the whipped cream along with the remaining pistachios. Dust with the rest of the icing sugar and serve with the apricot purée.