Step 1: Preheat the Oven

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour a 9×5-inch loaf pan or line it with parchment paper.

Step 2: Prepare the Batter

1. Mix Dry Ingredients: In a bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon. Set aside.
2. Cream Butter and Sugars: In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
3. Add Eggs and Vanilla: Beat in the eggs one at a time, then stir in the vanilla extract.
4. Combine with Dry Ingredients: Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk. Mix until just combined.
5. Fold in Apples: Gently fold in the diced apples.

Step 3: Prepare the Cinnamon Swirl

1. Mix Cinnamon Sugar: In a small bowl, combine the granulated sugar and ground cinnamon for the swirl.

Step 4: Assemble the Loaf

1. Layer the Batter: Pour half of the batter into the prepared loaf pan. Sprinkle half of the cinnamon sugar mixture over the batter.
2. Add Remaining Batter: Pour the remaining batter on top, then sprinkle the rest of the cinnamon sugar mixture over the top.
3. Swirl: Use a knife or toothpick to gently swirl the cinnamon sugar into the batter for a marbled effect.

Step 5: Bake

1. Bake the Loaf: Bake in the preheated oven for 50–60 minutes, or until a toothpick inserted into the center comes out clean.
2. Cool: Remove from the oven and let the loaf cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.

Step 6: Serve

1. Enjoy: Slice and serve the loaf warm or at room temperature. It’s delicious on its own or with a spread of butter!