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. Place dollops of the apple butter on top of the cake, then carefully smooth it 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. (Be sure to keep the leftover juice for the glaze!)
Top the apples with the streusel. Bake for 40-50 minutes, until a toothpick inserted in the center of the cake comes out clean or with only a few moist crumbs.
Let the cake cool in the pan until no longer too hot to touch, then use the parchment to lift the cake out of the pan.