Preheat oven to 180°C/350°F (160° fan-forced).

Cut round paper - Cut a round piece of baking / parchment paper, the size of the pie dish (to prevent skin forming on custard). (Note 5)

Blitz - Break up Oreos roughly by hand and place in a food processor. Blitz into crumbs (~10 sec). Add butter, blitz to mix through. Mixture should resemble wet sand. (No food processor? Bash in ziplock bag with a rolling pin).

Press - Pour into a 23cm / 9" pie dish. Spread crumbs out and press firmly into the base and up the walls using your hands, spatula or something flat.

Bake for 10 minutes. Remove from oven - the crust will be slightly puffed. Press down gently using a rubber spatula (makes it even crustier!) then allow to cool on the counter before filling.

Whisk dry, then wet - Place cornflour, sugar and salt in a large saucepan. Whisk to combine. Add milk, cream and yolks. Whisk to combine.

Heat to thicken - Turn heat onto medium high. As the mixture warms up, whisk every now and then, but not constantly. As the liquid starts to get hot at around the 3 - 5 minute mark (you'll see steam), turn the stove down to medium low and start to whisk constantly. You will feel and see the mixture start to thicken. Got lumps? See Note 6!

45 second whisk - When you see slow, lazy bubbles (~6 minutes, you will need to pause whisking to see bubbles), whisk constantly for 45 seconds then take it off the stove.

Chocolate and butter - Add butter, chocolate and vanilla. Whisk until chocolate melts and filling is smooth.

Pour hot filling into pie crust, filling it right to the top, smooth surface. Gently place round baking paper on surface. (Surplus custard? Note 7)

Cool on the counter for 2 hours then refrigerate for 12+ hours to allow the custard to fully set.

Whipped cream - Beat whipped cream ingredients in a bowl on high for 2 to 3 minutes until softly whipped.

Topping - Carefully peel back paper. Pile on the whipped cream, then grate chocolate across the surface.

Serving - Keep the pie in the pie tin. Cut and serve!