First, make the pastry cream. In a large mixing bowl, whisk together the egg yolks, caster sugar and cornflour into a very thick yellow paste. Set aside.
Measure 4 tablespoons of water into a small heatproof bowl, then sprinkle the gelatine on top (it’s important to do it this way round!). Mix well and leave the gelatine to bloom for 10 minutes. Then gently warm in a microwave, or place the bowl over a pan containing a little simmering water until the gelatine has fully melted.
Heat the milk and vanilla paste in a saucepan to just below boiling. Pour a quarter of the hot milk over the egg-yolk paste, whisking vigorously. Add another quarter and whisk again. Then pour everything back into the saucepan of milk.
Cook over a high heat for 1-2 minutes, whisking continuously until it resembles thick custard, then whisk through the gelatine. Pour it into the bowl of a stand mixer, cover with cling film pressed onto the surface to prevent a skin forming and put in the refrigerator for 2 hours to set.
Next, make the crust. Preheat the oven to 200°C fan/220°C/425°F/Gas mark 7 and brush a 22cm (8½in) fluted loose-bottomed deep-sided pie tin with oil.
Melt 80g (2¾oz) of the butter in a large saucepan, then add the light brown sugar. Cook over a medium–high heat for 3-4 minutes, whisking well until it forms a runny caramel. Stir through the cinnamon, then remove from the heat and quickly stir through the crushed cornflakes.
Tip them into a large mixing bowl – reserving 2 tablespoons to decorate – and allow to cool.
Blitz the Oreo cookies in a food processor, then add to the cooled cornflakes with a pinch of salt. Melt the remaining 40g (1½oz) of butter, pour into the bowl and mix everything to form a damp rubble.
Scrape the mixture into the tin then use the bottom of a flat-sided cup to compress and very tightly pack the crust into the base and sides of the tin – you want a thick rim with a deep cavity for the cream. Place on a baking sheet and bake for 10 minutes. If the crust has sunk a little, very carefully use the bottom of the cup to compress back into shape while warm. Leave to cool completely in the tin – it will harden as it does.
When ready to assemble, remove the chilled pastry cream from the refrigerator – it will be the texture of set jelly. Whisk in the stand mixer for 2 minutes to loosen, then gradually whisk in the very soft butter, dark roasted peanut butter and 1 teaspoon of salt.
Spoon the peanut butter cream into the cooled crust, smoothing over the top. Put in the refrigerator for 8 hours to set. Top with whipped cream, drizzle over extra peanut butter and decorate with the remaining cornflake crumbs and extra peanuts, if you like.