Topping: In a small bowl toss together sugar, flour butter and cinnamon until crumbly; set aside.
Muffins: In a large bowl whisk together brown sugar, oil, egg, cinnamon, and vanilla until smooth. In a separate bowl, sift together flour, soda and salt. Stir oil mixture alternately with buttermilk. Fold in apples, mixing just until combined. Spoon into greased muffin cups filling 3/4 full. Sprinkle topping over evenly. Bake at 350º for 25-30 mins or until golden brown and tops spring back.