Preheat oven to 350 degrees F. In a large bowl combine flour, brown sugar, and butter. Beat at low speed until crumbly. Stir in nuts. Press 2-¾ cups of crumb mixture into bottom of ungreased 9x13x2-inch baking pan.
To remaining mixture add cinnamon, baking soda, salt, sour cream, vanilla, and egg. Mix well. Stir in apples. Spoon evenly over base.
Bake for 25 to 35 minutes or until wooden pick inserted in center comes out clean. Cut into squares, spread caramel frosting atop squares while hot, and serve warm with whipped cream or ice cream.
Melt butter; add sugar and a dash of salt. Stir until combined, add milk, and bring to a boil. Remove to a small mixing bowl and let cool for 10 minutes. Then add vanilla and confectioners’ sugar until spreading consistency. Spread on hot squares.