Preheat your oven to 350°F (175°C).
Line a 9″x13″ pan with parchment paper and set aside.
In a small bowl, combine all the crumb ingredients except the butter, and whisk to combine. Add the cubed butter and mix using your hands or a pastry cutter until crumbly. Set aside.
In a large bowl, beat together the cold butter and white sugar until light and fluffy, scraping down the sides of the bowl with a rubber spatula as needed.
Add the eggs and vanilla extract and beat until fully combined. Then add the baking powder, baking soda, and sea salt, and mix again.
Add 1 cup of flour at a time, alternating with 1/2 cup of sour cream, beating on low speed until just combined. Be careful not to overmix. Repeat until all the flour and sour cream have been added.
Add your chopped apples and fold them into the batter with a rubber spatula or wooden spoon until just combined.
Spoon half of the batter into your prepared pan and spread it out evenly to cover the bottom.
Sprinkle half of the brown sugar crumb mixture over the batter as evenly as possible.
Spoon the remaining batter over the crumb layer and gently spread it out, doing your best to keep it as evenly spread as possible. Be sure to completely cover the crumb layer with the batter.
Finally, sprinkle the remaining crumb mixture evenly over the top.
Place the finished coffee cake into the preheated oven and bake for 55–60 minutes, or until the top is a deep golden brown and a toothpick inserted into the center of the cake comes out clean.
Remove from oven and allow to cool to room temperature before slicing and serving.
Enjoy!