Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan and set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon. Set aside.
In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Beat in the eggs one at a time, then stir in the vanilla extract.
Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk, starting and ending with the dry ingredients. Mix until just combined.
Gently fold in the diced apples and nuts (if using) until evenly distributed.
Pour half of the batter into the prepared loaf pan. Sprinkle with a layer of cinnamon sugar (1 tablespoon of sugar mixed with 1/2 teaspoon of cinnamon). Pour the remaining batter on top and finish with another layer of cinnamon sugar.
Use a knife or skewer to swirl the batter gently for a marbled effect.
Bake in the preheated oven for 55-65 minutes or until a toothpick inserted into the center comes out clean.
Allow the bread to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.