In a small bowl, whisk together the flour, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment (or with a hand mixer, in a large bowl) beat together the butter and sugar for 3-4 minutes, until light and fluffy.
Use a rubber spatula to scrape down the sides of the bowl. Add in both eggs and the vanilla. Beat until well combined (~1 min.)
Add in half of the flour mixture, and mix by hand just until the flour is incorporated. Add in all of the sour cream and milk and mix until combined. Add in the rest of the flour, and lightly fold the mixture together just until all of the flour is incorporated.
Pour half of the cake batter into the prepared cake pan and smooth out the top. Pour the cheesecake over the cake and carefully smooth it out into an even layer.
Pour the remaining cake batter into the pan and smooth it out into an even layer. Place the apples on top of the batter, making sure to leave out the juice from the apples.
Top the apples with the streusel. Bake for 55-65 minutes, until a toothpick inserted in the center of the cake comes out clean or with only a few moist crumbs and the cake is no longer jiggly in the center.
Let the cake cool in the pan until no longer too hot to touch. Once the cake is at room temperature, slice, serve and enjoy!