Preheat oven to 350°F (175°C). Prepare a 9-inch round baking pan with non-stick spray or butter and flour.
In a large bowl, beat softened butter with granulated sugar and brown sugar until light and fluffy, about 2-3 minutes.
Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
In a separate bowl, whisk together flour, baking soda, cinnamon, and salt.
Gradually add dry ingredients to wet mixture on low speed. Mix just until combined.
Fold in chopped apples and walnuts gently with a spatula.
Pour batter into prepared pan, spreading evenly.
Bake for 40-45 minutes until toothpick inserted in center comes out clean.
Cool in pan for 10-15 minutes before transferring to wire rack.