Preheat the oven to 350°F and grease a 9x5 inch loaf pan, then line the bottom with parchment paper.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon.
In a large bowl, beat the softened butter and granulated sugar until light and fluffy.
Add the eggs one at a time, mixing after each one, then stir in the vanilla.
Add the dry ingredients in thirds, alternating with the sour cream and milk. Mix gently and stop when just combined.
In a small bowl, toss the diced apples with brown sugar and cinnamon.
Spread half the batter into the pan, sprinkle half the apple mixture on top, then repeat with remaining batter and apples.
Use a butter knife to gently swirl through the batter.
Bake for 50 to 60 minutes until golden and a toothpick comes out with a few moist crumbs. Let the bread cool for 15 minutes before removing from the pan.
Whisk together powdered sugar, milk, and vanilla, then drizzle over the warm loaf.