Prepare topping by combining butter, brown sugar, flour and cinnamon in small bowl. Set aside. Peel and chop apples. Set aside. In mixing bowl, combine brown sugar, oil and egg. In a large measuring cup, combine buttermilk, soda, salt and vanilla. Blend flour into sugar and oil mixture alternately with buttermilk mixture. Stir just until combined. Fold in apples.
Spoon batter into greased muffin tins. Sprinkle with topping. Bake at 325 for 30 minutes. Cool for 10 minutes, then serve.