Place a rack in middle of oven; preheat to 350°. Line bottom of pan with a parchment paper round and lightly coat bottom and sides of pan with nonstick vegetable oil spray or room-temperature unsalted butter.

Toast ½ cup raw walnuts on a small rimmed baking sheet, tossing halfway through, until golden brown, 6–8 minutes. Let cool, then coarsely chop. Set aside.

Whisk 1 cup (200 g) granulated sugar and ½ cup (1 stick) unsalted butter, room temperature, in a medium bowl until smooth and creamy, about 2 minutes.

Add 2 large eggs and whisk vigorously until fluffy, about 2 minutes.

Add ½ cup sour cream and 2 Tbsp. apple cider vinegar and whisk to combine.

Add 1 tsp. ground cinnamon, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, 1 tsp. baking powder, and 1 tsp. vanilla bean paste or vanilla extract and whisk again until incorporated.

Add 2¼ cups (281 g) all-purpose flour and, using a large rubber spatula, fold in until no large dry streaks remain.

Add 3 Honeycrisp or Fuji apples (about 1½ lb.), peeled, cored, cut into ½" pieces, and gently mix until each piece is coated in batter.

Scrape batter into prepared pan and smooth surface with an offset spatula or a spoon.

Mix ½ cup (100 g) dark brown sugar, remaining 1 tsp. ground cinnamon, remaining ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt, and reserved walnuts in a small bowl to combine.

Sprinkle mixture evenly over cake.

Bake cake until top springs back when lightly pressed in the center or a tester inserted into the center comes out with only few moist crumbs attached, 45–55 minutes.

Transfer pan to a wire rack and let cake cool in pan before slicing.