Add the all-purpose flour and baking soda to the bowl. Whisk or fold with a spatula just until no flour lumps remain. Do not overmix.
Pour the batter into your prepared 9x9-inch pan and smooth the top with a rubber spatula.
Bake at 180°C (350°F) for 40 to 45 minutes, until a toothpick inserted into the center comes out clean with a few moist crumbs. Let the cake cool completely in the pan on a wire rack for at least 1 hour before frosting.
Add the room-temperature cream cheese and beat until smooth and creamy.
Spread the frosting over the completely cooled cake using an offset spatula. Dust with ground cinnamon before slicing.
Notes
Apple Selection: Use firm, tart apples like Granny Smith, Honeycrisp, or Braeburn for the best flavor balance and texture.
Grating Apples: Shred apples using the smaller holes of a box grater so the pieces melt into the batter without creating soggy pockets. Leaving the peel on is fine and adds extra moisture.
Measuring Flour: Fluff your flour with a spoon, scoop it into the measuring cup, and level it off with the back of a knife. Packing the flour will result in a dense cake.
Room Temperature Ingredients: Ensure the eggs, sour cream, and cream cheese are at room temperature. Cold ingredients will cause the batter to mix unevenly and can make the frosting lumpy.
Cooling Time: Allow the cake to cool completely for at least 1 hour before spreading the cream cheese frosting, or the whipped frosting will melt.
Storage: Keep the frosted cake covered in the refrigerator for up to 4 days. You can serve it chilled or let it sit at room temperature for 15 minutes before serving.