- Preheat oven to 375°F. Line muffin pan with paper or foil liners.
- Add the brown sugar, 1/4 cup flour, pinch of salt, and cinnamon to a large mixing bowl and beat briefly to blend, with mixer fitted with paddle attachment. Add 2 tablespoons margarine and beat on low until crumb mixture forms. Work the oats in with your hands. Pour into a small bowl and set aside.
- Return mixing bowl to mixer and add 2 tablespoons margarine, sugar and vanilla extract and beat until light and fluffy. Add egg and beat to combine, scraping sides of bowl.
- Add the flours, baking powder, baking soda, and 1/2 teaspoon salt to a 4-cup measure, and whisk to blend.
- Add the flour mixture all at once to the mixing bowl, along with the sour cream and buttermilk. Beat on low speed just until blended (scraping down the sides of bowl after 5 seconds). Stir in the apple chunks.
- Spoon a slightly heaping 1/8 cup of batter into each muffin cup. Sprinkle the topping evenly over the top of each muffin. Bake for about 20 minutes or until muffins are lightly browned and top springs back after being pushed.