Whisk together all the custard ingredients in a medium sized saucepan. Cook over medium-high heat, whisking constantly until mixture coats the back of your spoon.
Allow to cool and then chill thoroughly in the fridge overnight. Meanwhile make the mix-ins!
For the crumble:
Toss the cookie crumbs with the butter and toast at 375F for 10 minutes or until golden. Sprinkle with the sea salt and let cool.
For the apples:
Toss the apples with the cinnamon and cook with the remaining butter until soft.
To assemble:
When the ice cream is chilled, churn according to your maker's instructions. Layer in your container with the cookie crumbs and apples. Eat right away or freeze for a few hours to firm up.