Preheat your oven to 350°F (175°C) and position a rack in the center. Grease and line a 9-inch springform pan with parchment paper.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
Toss your diced apples with 1 tablespoon of the flour mixture to prevent them from sinking to the bottom of the cake.
In a small bowl, combine the melted butter, brown sugar, cinnamon, and flour until it resembles a thick paste for the cinnamon swirl.
Using an electric mixer, beat the softened butter and granulated sugar on medium-high speed for 3-4 minutes until light and fluffy.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract, then fold in the sour cream and applesauce.
Gradually add the dry ingredients to the wet mixture, mixing on low speed just until combined. Do not overmix.
Gently fold in the flour-coated apple pieces, distributing them evenly throughout the batter.
Pour half the batter into your prepared pan. Spoon half of the cinnamon mixture over the batter and use a knife to swirl it gently. Add the remaining batter and top with the rest of the cinnamon mixture.
Create spirals by dragging a knife through the batter in a circular motion from the center outward, mimicking the look of a cinnamon roll.
Bake for 45-50 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs. If the top browns too quickly, tent loosely with aluminum foil after 30 minutes.
Allow the cake to cool in the pan for 15 minutes, then remove the sides of the springform pan.
While the cake continues cooling, make the glaze by whisking together powdered sugar, milk, and vanilla until smooth. Drizzle generously over the warm cake, allowing it to seep slightly into the top layer.