For the apples:Peel and cut the apples to bite size slices, roughly about ¼ inch thick. Toss together sliced apples, sugar, and cinnamon. Set aside.
For the crumb topping:Mix together granulate sugar, brown sugar, and salt in a medium bowl. Stir in melted butter with a fork. Add the flour and cinnamon. Crumble with fork until no more flour is visible. Set aside.
For the Cake:Preheat oven to 350°F.Combine dry ingredients. Whisk together flour, baking powder and baking soda, cinnamon, and salt.
Cream butter and sugar. In a large bowl, cream together butter and both sugars until fluffy and lighter in color.
Add eggs, one at a time, mixing until fully incorporated after each addition.
Add buttermilk and dry ingredients alternately.Combine vanilla extract and buttermilk. Add half of the buttermilk mixture to the creamed butter mixture. Mix till combined. Add half of the dry ingredients (flour mixture). Mix until just combined (by hand or on low speed if by mixer). Repeat until all ingredients are combined and no more flour is visible. ***Cake batter will be thick.
Assemble. Pour batter into a parchment lined 9x13 cake pan. Spread evenly with a spatula or the back of a spoon. ***Have excess parchment flap to remove the cake painlessly.Sprinkle prepared apples and crumb topping over the top of the batter. Keep some of the crumbs larger to have more crunch.Pat the toppings down slightly to keep it adhered to the cake after baking.
Bake the cake for about 20-25 minutes or until a toothpick inserted comes out with a few crumbs. *Optional (for additional color and caramelization):Right before the cake is finished, turn the oven to 400°F broil for 2 minutes.
Cool. Remove cake from oven and cool on a wire rack until comfortable for handling. Remove cake from pan and let cool completely before cutting.
Serve. Enjoy the cake at room temperature or slightly warmed with a glass of milk and/ or a perfect drizzle of buttery caramel sauce!