1/3 cup brown sugar (not packed)
1 teaspoon ground cinnamon
2/3 cup white sugar
1/2 cup butter, softened
1 1/2 teaspoons vanilla extract
1 1/2 cups all-purpose flour
1 3/4 teaspoons baking powder
1/2 cup milk
1 apple, peeled and chopped
Preheat oven to 350 degrees. Grease and flour a 9 x 5-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 until smooth and creamy. Beat in eggs, 1 at a time, until incorporated; 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 apples and half the brown sugar-cinnamon mixture. Lightly pat apple mixture into batter.
Pour the remaining batter over apple layer; top with remaining apples and add more brown sugar/cinnamon mixture. Lightly pat apples into batter; swirl brown sugar mixture through apples using a finger or spoon.
Bake in the preheated oven until a toothpick inserted in the center of the loaf comes out clean, 30 to 40 minutes.