Preheat the oven to 350 degrees Fahrenheit and line an 8x8 cake pan with parchment paper. Set aside.
Add the filling ingredients to a pan. Bring to a boil, then lower and let simmer for about 8 minutes, or until the apples have softened a bit but still have a bite. Set aside to cool.
Combine the dry ingredients (flour, baking powder, baking soda, salt, and pumpkin pie spice) and mix together. Set aside.
Add the butter and sugar to a mixer fitted with the paddle attachment. Mix on low, and increasing to about medium, until creamed together and fluffy. Do not overmix.
With the mixer on low, add one egg and mix until combined. Add the second egg and mix until combined. Then add the vanilla and apple butter. Once combined, slowly add the milk. The batter will look curdled. This is normal.
On low, add the dry ingredients about ½ cup at a time, waiting for it to be incorporated before adding the next ½ cup. Once everything is adding to the mixing bowl, shut off the mixer and finish mixing with a spatula. You don't want to overmix this!
Pour ½ of the batter into the cake pan, spreading it out. Then add the apple filling and spread it out. Pour in the rest of the batter and spread it, making sure to cover the apples (it's okay if a few peak through!)
Add the sliced apples to the top if using (I do mine in 3 rows), then sprinkle the brown sugar, pumpkin pie spice, and salt.
Bake in the preheated oven for about 50-55 minutes, or until set. Remove and let rest in the cake pan for 15 minutes before moving to a wire rack. I'd say let it cool completely, but I've never been able to do that myself. HEHE.