Creme Patissiere:

Infuse milk: Place milk, vanilla seeds, used beans, and 50g sugar in a saucepan over medium-high heat. Bring to just before boiling. Remove from heat, cover, and steep for 10 minutes.
Yolk mixture: Whisk egg yolks, egg, and remaining sugar. Add cornflour until smooth.
Temper eggs: Gradually add about half the milk to the yolk mixture, whisking continuously.
Thicken custard: Return mixture to saucepan; cook over medium-low heat, stirring, until it thickens (3-5 minutes). Remove from heat and whisk vigorously if lumpy.
Finish custard: After seeing big bubbles, whisk on the stove for another 20 seconds. Whisk in butter until incorporated, strain, and cool with cling wrap on surface.
Crust:

Keep pastry as frozen as possible. Cut base and sides to fit a 20cm/8″ springform pan, not using the base of the pan.
Line pan: Butter pan and line with a baking paper strip. Quickly line with pastry, sealing edges with water.
Prepare to bake: Prick base with fork. Freeze prepared crust.
Bake:

Blind bake: Preheat oven to 220°C/430°F. Line crust with baking paper and beads; bake 25 minutes. Remove beads, bake additional 5 minutes.
Fill and bake: Lower oven to 200°C/390°F. Fill with custard, bake 65 minutes until set but wobbly.
Serve: Cool for 4 hours, refrigerate 12+ hours. Serve at room temperature.