Preheat your oven to 350°F (175°C) and grease a 9-inch round pan.
In a bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt.
Cream together butter and sugar in a separate bowl until fluffy.
Add eggs, one at a time, beating well after each. Stir in vanilla.
Add dry ingredients in two parts, alternating with sour cream. Mix until just combined.
Gently fold in the chopped apple.
For the cinnamon swirl: Mix sugar and cinnamon in a small bowl.
Pour half the batter into the pan, sprinkle with half the cinnamon mixture, and top with the rest of the batter.
Swirl the batter with a knife to mix the cinnamon into the cake.
Bake for 35-40 minutes, or until a toothpick comes out clean.
Let it cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.