Flan Pâtissier (French Custard Tart)

Bahan-bahan (4 porsi)

• 1 litre / 1 quart milk (, full fat)
• 2 vanilla beans (, seeds scraped, beans reserved (Note 1))
• 1 cup caster sugar (, separated)
• 120 g / 4.2 oz egg yolks ((~6 to 7 large eggs, 100 ml, Note 2))
• 1 large egg ((55 - 60g / 2 oz))
• 7 tbsp (70g) cornflour/cornstarch ((scoop and level, Note 3))
• 50 g (5 tbsp) unsalted butter (, cut into 1cm cubes (cold))
• 2 sheets butter puff pastry, FROZEN ((25cm / 10" squares, 185g/6oz each) (Note 4))
• 1 egg yolk (, whisked)
• Butter (, for greasing)