Preheat and prep pan: Preheat the oven to 350°F and grease a 9x5-inch loaf pan, or line it with parchment paper for easy removal.
Whisk wet ingredients: In a large bowl, whisk together the eggs, brown sugar, oil, buttermilk, and vanilla until smooth and pale, about 1 minute.
Combine dry ingredients: In a separate bowl, stir together the flour, baking soda, salt, and 1 teaspoon of the cinnamon.
Fold batter together: Add the dry ingredients to the wet mixture and fold gently just until no dry streaks remain; do not overmix or the loaf will turn dense.
Toss the apples: Toss the diced apples with 1 tablespoon of flour so they stay suspended in the batter instead of sinking, then fold them in gently.
Make the cinnamon swirl: Mix the granulated sugar with the remaining 1 teaspoon of cinnamon in a small bowl.
Layer the batter: Pour half the batter into the prepared pan, sprinkle with half the cinnamon sugar, then top with the remaining batter.
Swirl and top: Sprinkle the remaining cinnamon sugar on top and drag a knife through the batter in a figure-eight pattern to create a marbled swirl.
Bake the loaf: Bake for 50 to 60 minutes, until the top is deep golden brown and a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool before slicing: Let the bread cool in the pan for 15 minutes, then transfer it to a wire rack to cool for at least 30 minutes before slicing.