Start out by making the custard. To a mixing bowl, whisk together cornstarch, sugar, and salt. Then add all the yolks and beat until it gets lighter in color. Set aside.

To a pot over medium heat, add the milk and vanilla extract. If using vanilla bean, add now. Combine together and heat it just until it starts to bubble on the edges. You do not want to boil it or it will be too hot.

Take off the heat and slowly add a little in at a time to the batter. This is so you don't shock the egg yolks and cook them. Keep on slowly integrating the warm milk until it's all whisked in and very smooth.

Then transfer back to the pot and heat over medium-low heat. Continuously whisk together for a few minutes until it starts to thicken up. Immediately take off the heat and add the butter. Mix together until the butter is melted in and incorporated. Transfer the custard to a bowl.

Preheat oven to 325°F.

To a small bowl, add the melted butter, white sugar, all-purpose flour, oats, and a pinch of salt. Mix together with a rubber spatula or your hands until the consistency resembles cookie dough.

Line a sheet pan with parchment paper and crumble the dough across the tray.

Bake for 10 to 12 minutes, just until golden on the edges. Take out of the oven and allow to cool.

To a pan over medium heat, add the butter, apples, light brown sugar, cinnamon, salt, vanilla extract, and lemon juice.

Mix together and allow to simmer on low heat for 6 to 8 minutes until the apples are soft when poked with a fork.

Combine the water and cornstarch in a small bowl to make the cornstarch slurry and add this to the pan. Mix together to thicken the filling and then take off the heat and transfer to a bowl.

To a large cup or bowl, add a serving of the apple pie mix, then add shortbread crumble on top, and finish it off with the hot custard to enjoy!