Combine flour, brown sugar, cinnamon and salt.
Cut in butter until mixture resembles coarse crumbs.
Stir in nuts and set aside.
Preheat oven to 350°F.
Butter a 10-inch round spring-form pan (or a 9-inch square baking pan).
Whisk together flour and baking powder and set aside.
Cream together butter, sugar, cinnamon and salt until light and fluffy. Blend in eggs and vanilla.
Gradually blend in flour mixture.
Stir in apples.
Transfer batter to prepared pan. Sprinkle topping evenly over apple mixture.
Bake 45-50 minutes or until wooden skewer inserted into center of cake comes out clean.
Allow to cool 15 minutes and remove sides of pan. Serve warm or at room temperature.