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

In a small bowl, whisk together the 1/3 cup of granulated sugar and 1 tablespoon of ground cinnamon for the swirl. Set aside.

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

In a large mixing bowl, beat the softened butter and 3/4 cup of granulated sugar together on medium speed until smooth and fluffy, about 2 minutes.

Add the eggs one at a time, beating well after each addition, then stir in the vanilla extract.

Alternate adding the dry ingredients and the sour cream/milk mixture to the wet ingredients, mixing just until combined. Do not overmix.

Gently fold the diced apples into the batter using a spatula.

Spoon half of the batter into the prepared loaf pan and smooth the top. Sprinkle half of the cinnamon-sugar mixture evenly over the batter.

Add the remaining batter on top, smooth it out, and sprinkle the rest of the cinnamon-sugar mixture over the surface. Use a butter knife to swirl the batter and create a marbled effect.

Bake for 50 to 60 minutes, or until a toothpick inserted into the center comes out clean. If the top is browning too quickly, tent it loosely with aluminum foil at the 35-minute mark.

Allow the bread to cool in the pan for 15 minutes, then transfer it to a wire rack to cool completely before slicing.