Prep loaf pan: Preheat the oven to 350 F. Grease an 8.5 x 4.5-inch loaf pan and line with parchment paper. Set aside.
Make the swirl: In a small bowl, mix together brown sugar, apple butter and ground cinnamon and set aside.
Combine dry ingredients: In a medium bowl, whisk together flour, salt, baking powder and cinnamon and set aside.
Whisk wet ingredients: In a separate bowl, cream together granulated sugar, brown sugar and butter until light and fluffy. Whisk in eggs, milk and vanilla extract until smooth.
Mix batter: Add dry ingredients to wet ingredients and fold together gently until batter is evenly combined. Gently fold apples into the batter.
Pour and swirl: Pour half of the batter into the loaf pan. Spoon out ½ of the apple butter swirl mixture onto the first half of the batter and swirl together gently with a knife. Pour the remaining batter on top. Top with the remaining apple butter mixture and swirl the top together gently.
Bake, cool and slice: Bake for 50-55 minutes, or until the top is golden and a toothpick inserted in the center is removed clean. Allow to cool for 20-30 minutes before slicing.