In a saucepan over medium heat, combine the milk, double cream, and vanilla extract until hot but not boiling. Whisk whole egg, egg yolks, caster sugar, and cornflour in a bowl until smooth. Slowly pour the milk mixture into the egg mixture, whisking continuously. Return to the saucepan and cook until thickened, about 5-7 minutes. Cool slightly, cover with cling film.

On a flour-dusted surface, roll out the ready-rolled puff pastry to about 2-3mm thick. Cut out 10cm discs with a round cutter. Grease a muffin tin with butter, press each pastry disc into the muffin cups, and prick the bases with a fork.

Spoon custard into each pastry case, filling them about 3/4 full. Top with sliced apricots. Chill filled tarts in the fridge for about 30 minutes.

Preheat oven to 200°C (180°C fan). Line a baking tray with parchment paper. Sprinkle chilled tarts with demerara sugar and bake for 25-30 minutes, until pastry is golden brown and custard is set.

Remove from oven, brush tops with melted apricot jam, and let cool in the tin for 10 minutes before transferring to a wire rack.