Combine dry ingredients and blend well. Melt butter, stir in sour cream, beat in eggs. Mix in with dry ingredients until smooth and satiny. Stir in apple. Scrape batter into prepared pan (buttered and floured) smoothing top level. Top with crumb topping and bake 1 hour @ 350 F. Cool in pan 20 minutes before cutting.
Crumb Topping:
1/4 C sugarMix dry ingredients and cut in cold butter to resemble coarse crumbs.