1. Preheat oven to 350 degrees F. Spray a 9-by-5-inch loaf pan with nonstick spray or line with parchment paper, for easy removal.
2. Mix brown sugar and 1 teaspoon cinnamon together in a bowl; set aside. Toss apples with 2 tablespoons granulated sugar and remaining teaspoon cinnamon in another bowl.
3. Beat butter and remaining 2/3 cup granulated sugar together in another medium bowl with an electric mixer until smooth and creamy, about 5 minutes. Beat in eggs, 1 at a time, until blended in; add vanilla extract.
4. Whisk flour and baking powder together in another bowl, then add to creamed butter mixture and beat until blended. Mix 1/2 cup milk into batter until smooth.
5. Pour half the batter into the prepared loaf pan; add half the chopped apple mixture.
6. Sprinkle half of the brown sugar/cinnamon mixture on top of apple layer.
7. Pour the remaining batter over apple layer and top with remaining chopped apples, then the remaining brown sugar/cinnamon mixture.
8. Lightly pat apples into batter; swirl brown sugar mixture through apples using a knife or spoon.
9. Bake until a toothpick inserted in the center of the loaf comes out clean, approximately 1 hour and 10 minutes.
10. Let the loaf cool for about 15-20 minutes.

NOTE: You can omit the apples for a plain cinnamon swirl cake.

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