Add milk, vanilla seeds, reserved beans and 50 g sugar in a large saucepan over medium-high heat. Bring to a boil, while stirring to dissolve the sugar. Remove from heat, cover with the lid and leave to steep for 10 minutes.

Meanwhile, add the egg yolks, egg and remaining sugar in a large bowl and whisk vigorously to combine. Add cornflour and whisk until smooth and silky.

Slowly pour in about half the milk in a thin stream while whisking the eggs until combined.

Pour the egg-milk mixture back into the saucepan with the vanilla-infused milk. Whisk to combine. Place over medium-low heat, stirring constantly so the base doesn't set, until thick and consistent (in about 3 to 5 minutes).

When the custard is thickened and you see the first big bubbles pop up, whisk for a further 20 seconds then remove from heat. If needed, pause stirring for a few seconds to check for bubbles. Whisk butter in until it's fully incorporated.

Immediately use a fine mesh strainer and pour into a bowl. Remove the vanilla bean. Cover the bowl with cling wrap, ensuring it's touching the custard surface. Allow to cool on the counter for 3 hours then refrigerate for 12 – 24 hours.

Lay out the frozen puff pastry sheets. Use a pan as a guide to cut the base out of one puff pastry sheet. Cut three wide strips on the other sheet of puff pastry. Place the base and sides back in the freezer. Keep any offcuts for potential patching.

Remove the pan base from a 20 cm rectangle springform pan. Place the springform on a baking paper wide enough to cover the form. Butter and line the form sides with a strip of baking paper. Place the cake pan on a square sheet of baking paper on a plate. Remove the cut puff pastry from the freezer.

Working quickly, when the sides have thawed enough (but not completely) and are bendy, line the sides of the cake pan with puff pastry. Use water to seal temporarily pressing down. Brush the base with water and fit it into cake pan.

Use the back of a teaspoon to press the base into the corners. Then press enough so the now-thawing puff pastry fully seals with the sides. Pierce the base throughout the surface with a fork. Cover with cling wrap, and freeze for 4 hours.

Preheat oven to 220°C for 30 minutes.

Pick up the baking paper below the form and place it on a tray. Crumple 2 sheets of baking paper and fit into the form covering the puff pastry. Fill with baking beans and press to push the baking paper against the corners.

Blind bake 25 minutes. Lift the overhang paper to remove the beans carefully. Bake for further 5 minutes, then cool for 10 minutes.

Reduce the oven temperature to 180°C fan.

Remove the custard from the fridge and whisk to loosen. Transfer on top of the crust, filling up to 1cm below the form edge. Smooth  the surface, and brush the custard surface with egg yolk.

Bake for 1 hour, rotating the tray after the first 45 minutes. The flan tart will puff up during the last 15 minutes. Remove from the oven. It should be oozy with a caramelised top, but it will firm up as it cools. If the top isn’t caramelized, use a kitchen torch until it reached the desired colour.

Cool in room temperature for 4 hours without removing from the springform. Then transfer to a plate (still in the form), cover loosely with cling wrap, and refrigerate 20 hours.

To serve, remove from the fridge and leave in room temperature for 1 hour prior to serving. Cut into slices to serve.