Preheat oven to 375 degrees F (190 degrees C). Butter a 9 inch round cake pan or 9 1/2 inch springform pan.
Using a fork, work flour with brown sugar and butter in a large bowl until crumbly. Stir in nuts. Divide mixture in half. Evenly press half into pan bottom to form a crust.
Stir baking soda, cinnamon, and salt into remaining crumb mixture until blended, then make a well in the center. In a small bowl, lightly beat egg with sour cream and vanilla until smooth. Add to flour mixture, stirring just until combined. Fold in apples
Evenly spread batter over crumb mixture. Bake until a cake tester inserted into center comes out clean, about 1 hour and 20 minutes. If top gets too brown before cake is baked, lightly lay a piece of foil over top for last 10 to 20 minutes of baking.
Cool in pan on a rack. Serve with Brown Sugar Sauce. Cake keeps well at room temperature for a day. Covered, it freezes well and is easily cut while still frozen.