Start by preheating your oven to 350°F (175°C) and greasing a 9-inch round cake pan. Next, peel, core, and dice the apples, and set them aside. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

In a separate large bowl, cream together the softened butter and granulated sugar until light and fluffy. Beat in the eggs, one at a time, followed by the vanilla extract. Gradually add the dry ingredients to the wet ingredients, alternating with the sour cream, and mix until just combined. Gently fold in the diced apples and lemon zest.

To infuse your cake with lemon flavor, add the zest of one lemon to the batter. The lemon zest adds a bright, citrusy aroma and taste to the cake, complementing the sweetness of the apples.

Pour the batter into the prepared cake 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. Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.