For the cake, preheat oven to 350 degrees. Butter 9-inch-diameter springform pan. Combine flour, brown sugar, butter, and salt in large bowl. Using electric mixer, beat until mixture resembles coarse meal. Set aside 2/3 cup of mixture for topping. Beat pumpkin, sour cream, 2 tablespoons sugar, spice, and baking soda into remaining flour mixture, beating just until smooth. Beat in eggs. Transfer batter to pan. Scatter apples evenly over top. Sprinkle reserved topping over apples.