Preheat oven to 350°F. Grease and flour loaf pan.
Prepare streusel: mix flour, sugar, cinnamon, cut in cold butter, chill.
Whisk dry ingredients for batter.
Cream butter and sugar, add eggs & vanilla.
Alternate adding dry mix and sour cream.
Fold in diced apples.
Pour batter in pan, top with streusel.
Bake 55–65 min, cover if browning too fast.
Cool 15 min in pan, then fully on rack.