Got this from a friend this weekend. I love cinnamon, so this is a new favorite for me. I love it just as written. Some people have made comments that they felt this is better with about 1/4- 1/2 teaspoon salt. It's your call. Make it yours.
Preheat oven to 350. Grease and flour a 9x5 inch loaf pan. Mix brown sugar and cinnamon together in a bowl and set aside. Beat white sugar and butter together in a bowl using an electric mixer intil smooth and creamy. Beat in eggs, one at a time, until uncorporated; add vanilla extract.
Combine flour and baking powder together in another bowl; stir into creamed butter mixture. Mix milk into batter until smooth. Pour half the batter into the prepared loaf pan. Next add half the apple and half the brown sugar cinnamon mixture. lightly pat apple mixture into batter.
Pour remaining batter over apple layer; top with remaining apple and add more brown sugar cinnamon mixture. Lightly pat apples into batter; swirl brown sugar mixture through apples using finger or spoon.
Bake 40-50 minutes, until a toothpick inserted in center of the loaf comes out clean. (Cooking time may vary, one report of 60 minutes)