Preheat oven to 350. Spray a loaf pan with nonstick cooking spray and it set aside.
Combine the chopped apples, 2 tablespoons of dark brown sugar, and 1 teaspoon ground cinnamon in a bowl. Drizzle 1 teaspoon of melted butter over the top and mix.
Combine 1/2 cup brown sugar and remaining cinnamon in a small bowl and mix.
Cream the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment, or use an electric hand mixer.
Add the eggs one at a time and mix in between each, scraping down the sides as needed. Add the vanilla and mix.
Whisk the milk and sour cream together in a bowl or measuring cup.
Combine the flour, baking powder, and salt in a large bowl, then add it to the creamed butter and sugar. Mix on low and drizzle in the milk and sour cream mixture. Mix until just combined, scraping down the bowl as needed.
Transfer half the batter to the bottom of the prepared loaf pan and smooth it out. Top with half the apples and half the brown sugar-cinnamon mixture.
Top with the remaining batter, the rest of the apples, and the remaining cinnamon sugar.
Bake until golden brown, and an inserted knife comes out mostly clean. About 50 minutes.
Meanwhile, make the glaze by mixing the powdered sugar, 1 tablespoon of milk, and 1/2 teaspoon of vanilla extract until smooth.
Cool the cake on a wire rack until completely cool. Turn it out of the pan and drizzle with the glaze.