Preheat the oven to 350°F / 175°C. Butter and line an 8x4" loaf pan with parchment paper.
In a small bowl, whisk together the flour, baking powder, and salt. Set aside.
Add the butter and sugar to a medium sized mixing bowl and use a hand mixer or a stand mixer fitted with the paddle attachment to beat the mixture for 3-4 minutes, until it's light and fluffy.
Scrape down the sides of the bowl. Then, with the mixer running, beat in the eggs one at a time, beating for 30 seconds before adding in the next egg and the vanilla.
Add in half of the flour mixture and gently fold it into the wet ingredients just until the last streak of flour disappears.
Then, add in the milk, sour cream and gently fold them into the batter until just combined. Add in the remaining flour mixture and fold just until the last streak of flour disappears.
Spread half of the batter into your loaf pan, then sprinkle 1/3 of the spiced sugar mixture over the loaf, followed by all of the apples. Spread the remaining batter over the apples, then top it with the remaining spiced sugar mixture.
Bake for 50-65 minutes, or until a toothpick inserted in the center of the loaf comes out with only a few moist crumbs on it.