Preheat the oven to 180℃, spray or brush a 9 inch cake tin with oil and set aside.
In a large mixing bowl add the flour, corn flour, salt, cinnamon, nutmeg and stir together. Add the butter and rub between your fingers until it resembled damp sand.
Add the apples, sugar and stir together. Add the eggs, milk, vanilla and using a rubber spatula mix together until combined.
Transfer to the lined tin and bake for 40-45 minutes.
While this is happening make the custard by adding the eggs, sugar, vanilla, cornflour and milk in a sauce pot, whisk together until smooth. Place it on the stove on low to medium heat and start whisking - do not leave the stove and whisk constantly as the custard will start thickening fast. After 3-5 minutes it will get thick, take it off the heat and add the water and heavy cream, whisk until combined.
You know the cake is done when it has risen, the top is crusty and golden, a tooth ick inserted into the center comes out clean. Transfer to a platter and serve with the custard.