Preheat oven to 350℉/175℃/Gas Mark 4.
In a small bowl, mix together the apple filling: peeled and diced apples, brown sugar, and cinnamon. Set aside.
In a medium bowl, whisk the all-purpose flour, cinnamon, baking powder, and salt together. Set aside.
In a large bowl, cream the unsalted butter with the granulated sugar. Beat on medium speed for about 3 minutes, scraping down the sides of the bowl as needed.
Add the eggs, sour cream, and vanilla extract. Continue beating on medium speed until incorporated for about a minute.
With the beaters on low speed, slowly pour in the dry ingredients and beat until just mixed. Then add the milk and gently fold in.
Grease a 9x5 loaf pan and pour about half of the batter into the bottom of the pan. Spoon the apple filling onto the batter, then cover it with the rest of the batter.
Bake for 1 hour or until a toothpick inserted comes out clean. Let cool in the pan for 5 minutes, then transfer to a cooling rack to continue to let cool. Serve and enjoy!