Preheat oven to 400 degrees F. Spray a 12 cup muffin tin with nonstick spray.
Prepare the streusel: Combine the streusel ingredients in a bowl and set aside.
Prepare the muffins: In a medium bowl, whisk together the flours, cinnamon, baking powder, baking soda and salt. In a separate medium bowl, whisk together the buttermilk, sugar, oil, butter and egg. Add the wet ingredients to the dry ingredients and stir just until combined. Stir in the apple. Divide the batter evenly among the 12 muffin cups. Spoon streusel mixture on top.
Bake for 16 minutes, or until a wooden pick inserted into the center comes out fairly clean (with maybe a few crumbs sticking to it). Cool for a few minutes before removing the muffins from the pan.