Preheat oven to 350°F. Grease and flour a 9x5-inch loaf pan.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Set aside.

In a large bowl, cream together butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in vanilla.

Gradually add dry ingredients to the wet ingredients, alternating with buttermilk, mixing until just combined. Gently fold in apples.

In a small bowl, mix sugar and cinnamon for the filling.

Spread half the batter into the loaf pan, sprinkle with half of the cinnamon-sugar mixture, then top with remaining batter and the rest of the cinnamon-sugar.

Swirl a knife through the batter to create a marbled effect.

Bake for 50-60 minutes, or until a toothpick inserted in the center comes out clean. Cool in pan for 10 minutes, then transfer to a wire rack to cool completely.

In a small bowl, whisk together powdered sugar, milk, and vanilla until smooth.

Drizzle glaze over cooled bread, slice, and serve.