Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan or line it with parchment paper.

In a medium bowl, whisk together the flour, baking soda, baking powder, ground cinnamon, and salt.

In a large bowl, beat the softened butter and granulated sugar together until fluffy.

Add the eggs one at a time, followed by the vanilla extract. Mix until well combined.

Stir in the applesauce until fully incorporated.

Gradually add the dry mixture to the wet ingredients, mixing just until combined.

Gently fold in the diced apples.

In a small bowl, combine the brown sugar and ground cinnamon to create the swirl mixture.

Pour half of the batter into the prepared loaf pan, sprinkle half of the cinnamon swirl mixture on top, add the remaining batter, and top with the remaining swirl mixture.

Using a knife or skewer, gently swirl through the batter to create a marbled effect.

Bake for about 50-60 minutes, or until a toothpick comes out clean.

Allow the bread to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.