Preheat oven to 350 degrees F. Spray a 13-9-inch pan with cooking spray and set aside.
In a large mixing bowl, stir together the flour, brown sugar and walnuts until very well combined. Add the melted butter and use a fork to stir together until you have a very crumbly mixture.
Transfer 2 1/2 cups of the mixture to the prepared pan. Loosely pack the mixture to measure. Then use the back of a measuring cup to lightly press the crust into the pan.
To the remaining crumb mixture in the same bowl, add the sour cream, egg, vanilla, cinnamon, baking soda and salt. Whisk vigorously, or use a spatula to stir it together. Be sure it is smooth and very well mixed.
Add the chopped apple and mix again. Gently spoon the sour cream and apple mixture into the pan on top of the crust. Spread it evenly into the pan and gently smooth the top.
Bake for 30-40 minutes, until the cake is golden brown and it springs back when you lightly touch it.
Allow the squares to cool in the pan. Slice into 12 squares. Top each square with a dollop of whipped cream or a scoop of vanilla ice cream.