Preheat oven to 350 degrees F. Line a 9-inch cake pan with two pieces of crumpled up parchment paper. I wad them up like trash and then smooth them out. I find this makes it easier to mold them into the cake pan. If you prefer, you can also grease and flour the cake pan instead.
With a stand mixer fitted with paddle attachment (or electric hand mixer), beat together the softened butter, eggs, one cup of sugar and vanilla. Start on low speed then increase to high speed. Beat until pale and fluffy, 4 -5 minutes. I like to peel and chop my apples while my stand mixer is doing the work.
Scrape down the bowl with a rubber spatula. In a small bowl, whisk together the flour, baking powder and salt. Dump half the flour mixture into the butter mixture. Stir on low until just combined. Add the full amount of sour cream, mix again. Add the remaining flour and chopped apples and mix on low to medium low speed until just combined.
Pour the batter evenly into the prepared cake pan. I like using a large cookie scoop to help with this process. Use an offset spatula to smooth the top of the cake. In a separate small bowl, stir together the remaining 1/4 cup of sugar with the cinnamon. Evenly sprinkle the mixture over top of the cake.
Bake at 350 for about 40 minutes, until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for 20 minutes. Then use the parchment paper to gently lift it out of the cake pan and let finish cooling on a wire rack for 1 - 2 hours before slicing.