Prepare your oven and pan. Preheat your oven to 350°F (175°C) and line a 9×5 inch loaf pan with parchment paper, leaving some overhang for easy removal. Alternatively, grease and flour the pan thoroughly.
Mix the dry ingredients. In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and 1 teaspoon of the cinnamon until well combined.
Combine the wet ingredients. In a large bowl, beat the eggs, granulated sugar, and brown sugar until the mixture is light and slightly fluffy, about 2 minutes. Add the oil (or melted butter), buttermilk, and vanilla extract, whisking until smooth and fully incorporated.
Create the batter. Gradually add the dry ingredients to the wet mixture, stirring just until combined—overmixing will make your bread tough. Gently fold in the diced apples (and nuts if using) with a rubber spatula or wooden spoon.
Prepare the cinnamon swirl mixture. In a small bowl, mix the 1/3 cup of sugar with the remaining 2 teaspoons of cinnamon until evenly combined.
Layer the batter with cinnamon swirl. Pour half of the batter into the prepared loaf pan. Sprinkle about 3/4 of the cinnamon-sugar mixture evenly over the batter. Add the remaining batter on top, then sprinkle with the remaining cinnamon-sugar mixture.
Create the swirl effect. Using a butter knife or skewer, gently swirl through the batter in a figure-eight pattern, being careful not to overmix—you want distinct layers of cinnamon throughout the bread.
Bake until golden and set. Place the loaf in the preheated oven and bake for 50-60 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs (not wet batter). If the top begins to brown too quickly, loosely cover with aluminum foil after about 35-40 minutes.
Cool properly before slicing. Allow the bread to cool in the pan for 15 minutes, then use the parchment overhang to lift it out onto a wire rack to cool completely, at least 1 hour, before slicing. This resting period allows the texture to set properly.