Preheat the oven to 375-degrees F. Line two (9x5-inch) loaf pans with paper liners or spray with cooking spray, set aside.
In a large mixing bowl of a stand mixer or use an electric hand mixer, cream together the butter and sugar.
Add the eggs, vanilla, and sour cream and combine. Occasionally scrape the sides and bottom of the bowl with a rubber spatula. Add the flour, baking powder and salt, stirring to combine.
Spread half the batter into the prepared baking pans. Top with half of the apple mixture, sprinkle with half of the cinnamon sugar mixture. Spoon remaining batter over the apples, use a butter knife or rubber spatula to spread evenly. Top with remaining apples and cinnamon sugar mixture.
Place the baking pans on a half sheet pan, lined with parchment paper. (This helps catch any dropped batter and helps easily rotate pans.) Place into the oven.
Bake for 8-10 minutes and LOWER the temperature to 350-degrees F, baking for 60-65-minutes or until a toothpick when inserted comes out mostly clean, with a few moist crumbs. Towards the end of the baking process carefully rotate pans 180-degrees.
Remove loaves from the oven. Let cool in the baking pans for 5-10 minutes. Turn out of baking pans, place loaves on a wire cooling rack to cool completely.
In a small bowl, whisk together ingredients for the glaze; powdered sugar, vanilla extract and milk. While loaves are still warm, drizzle the glaze over the top. Let the loaves cool for a minimum of 30-minutes before slicing.