Preheat the oven to 350°F (175°C). Grease and flour a 9×9 inch square baking pan.
In a small bowl, toss the diced apples with the lemon juice to prevent browning. Set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, ground cinnamon, and salt.
In a large bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Add the eggs one at a time to the butter mixture, beating well after each addition.
Mix in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with the sour cream, beginning and ending with the dry ingredients. Mix until just combined.
Fold the diced apples gently into the batter.
Pour the batter evenly into the prepared baking pan and smooth the top with a spatula.
Bake in the preheated oven for 40-45 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.