Preheat your oven to 350°F (175°C) and grease a 9x5 inch loaf pan.

In a bowl, whisk together flour, baking powder, and salt. Set aside.

In a large bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3 minutes. Add eggs one at a time, then vanilla extract, mixing well after each addition.

Gradually add in the dry ingredients alternately with the milk, beginning and ending with the flour mixture, until just combined.

In a small bowl, mix sliced apples with cinnamon and brown sugar. Pour half the batter into the prepared loaf pan, layer with half the apple mixture, and swirl gently with a knife. Repeat with remaining batter and apple mixture.

Bake for 45-55 minutes, or until a toothpick inserted into the center comes out clean. Cool in the pan for 10 minutes before transferring to a wire rack to cool completely.