Preheat your oven to 350 F degrees. Lightly butter or oil your baking dish.
In a large bowl, combine the apples, brown sugar, cornstarch, and spices. Toss until the apples are evenly coated. Transfer the mixture to the prepared baking dish and pat into an even layer.
Whisk the flour, sugar, baking powder, baking soda and salt together in a large bowl. Using two forks (or a pastry cutter) cut in the butter until you have pea sized crumbs.
Whisk together the egg yolk and buttermilk. Slowly add the mixture to the flour and stir with a rubber spatula until evenly combined. Rest the dough for 10 minutes.
Spoon the dough all over the top of the apple filling. The filling should be mainly covered with some pieces filling showing through. Optional: Sprinkle extra sugar or cinnamon and sugar over the top.
Bake uncovered until the fruit is bubbling and the top is golden, about 40 to 55 minutes. If it looks like it's getting too brown, cover it loosely with aluminum foil. Cool for 10 to 15 minutes before serving.
Notes
See post for best baking apples. I used a 50/50 blend of Granny Smith and Honeycrisp apples.
To make ahead fully bake and cool before storing in the fright. Reheat at 300 F degrees until warmed through.