Sour Cream Apple Crumb Kuchen

For topping: Blend walnuts, brown sugar, flour and cinnamon in a processor until nuts are finely chopped. Add butter and cut in using the on/off button, just until clumps begin to form. (Can be made one day ahead. Cover and chill.) For filling: Mix apples, sugar and cinnamon in a medium bowl. For cake: Prehat oven to 350. Butter a 13" x 9" x 2" GLASS baking dish. Whisk sour cream and milk together in a small bowl to blend. Sift flour, baking powder, baking soda and salt into a medium bowl. Using an electric mixer, beat butter and both sugars in a large bowl until creamy. Add eggs, one at a time, beating after each addition. Mix in lemon peel, vanilla and lemon extract. Beat in dry ingredients alternately with sour cream mixture in 3 additions each, mixing just until smooth. Spread half of the batter in prepared pan. Spoon filling on top of batter. Spread remaining batter over filling. Sprinkle with topping. Bake cake until tester inserted into center comes out clean, covering top loosely with foil if browning too quickly, for about 45 minutes. Cool in pan on rack for at least 2 hours. Cut into squares.