In food processor, combine first 3 ingredients with 1/4 C butter. Process until mixture looks like coarse meal. Set aside. In large bowl, cream 3/4 C butter with sugar. Add eggs one at a time; stir in vanilla. Sift dry ingredients together. Add to creamed mixture alternately with plain yogurt, beginning and ending with dry ingredients. Pour 1-1/2 C batter into each of 2 buttered 9" x 5" loaf pans. Top each with 1/3 C streusel mixture and 1/4 C of apple. Divide remaining batter, streusel, and apple between pans. Cut with a knife to marble the ingredients. Bake at 350 °F for 55 minutes or until pick inserted in center comes out clean. Serve warm. Recipe freezes well. 2 loaves servings(s).