Preheat the oven to 350°F (175°C). Grease and flour a 9×9 inch square baking pan.
In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, ground cinnamon, and salt. Set aside.
In a large bowl, beat the softened butter and granulated sugar together until light and fluffy, about 3 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Add half of the dry ingredients to the butter mixture and mix until just combined.
Add the sour cream and mix until incorporated.
Add the remaining dry ingredients and mix until just combined, being careful not to overmix.
Fold in the diced apples gently until evenly distributed.
Pour the batter into the prepared baking pan and spread evenly.
Bake in the preheated oven for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and allow the cake to cool in the pan for 15 minutes.
Transfer the cake to a wire rack to cool completely before slicing into 12 pieces.