1. Preheat oven to 350 degrees F. Grease the bottom and 1/2 inch up the sides of an 9x5x3-inch loaf pan; set aside.
2. In a large bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Beat in sugar until combined. Add buttermilk and baking powder; beat until combined. Add eggs and vanilla; beat until combined. Add flour and salt; beat until combined. Stir in apple, nuts, and raisins.
3. Spoon batter into prepared pan; spread evenly. Sprinkle Streusel-Nut Topping over batter. Bake for 60 to 65 minutes or until a wooden toothpick inserted near the center comes out clean.
4. Cool in pan on a wire rack for 10 minutes. Remove from pan. Cool completely on wire rack. Wrap and store overnight before slicing.
5. Makes 1 loaf (14 servings)
6. *Note: To make 1/4 cup sour milk, place 3/4 teaspoon lemon juice or vinegar in a glass measuring cup. Add enough milk to make 1/4 cup total liquid; stir. Let mixture stand for 5 minutes before using.
7. Streusel-Nut Topping: In a small bowl, combine 1/4 cup packed brown sugar and 3 tablespoons all-purpose flour. Using a pastry blender, cut in 2 tablespoons butter until mixture resembles coarse crumbs. Stir in 1/3 cup chopped walnuts or pecans.