Crumb Topping: In a small bowl blend: 3 tablespoons all-purpose flourm 1/4 teaspoon cinnamon/ Cut in 2 tablespoons cold butter until mixture forms coarse crumbs.
Heat oven to 350°. Butter a bundt pan or souffle dish and sprinkle with bread crumbs, tapping out excess.
Combine dry ingredients and blend well.
Melt butter. Off heat, stir in sour cream and beat in eggs.
Beat the butter mixture into the dry ingredients and blend until batter is smooth and satiny. Stir in apple. Scrape batter into prepared pan, smoothing batter until it is level. Sprinkle with crumb topping.
Bake 1 hour until crumbs are crisp and lightly browned and a skewer inserted in center of cake comes out clean. If tube pan is used, start testing for doneness after 45 minutes.
Cool cake in pan 20 minutes.
Turn and knife between cake and sides of pan and gently invert cake onto counter.
Re-invert on a rack and cool thoroughly before serving.