In large bowl, stir together flour, sugar, baking powder, cinnamon, salt, and soda, set aside. In a small bowl, beat eggs, sour cream, and butter. Add all at once to dry ingredients along with apples and black walnuts. Stir just until moist. Fill well-greased muffin tins 2/3 full.
Streusel Topping:
Combine topping ingredients, sprinkle on unbaked muffins, bake at 400° preheated oven for 20 to 25 minutes. Makes 18.