Preheat oven to 325°F.

Grease and flour muffin tins or use paper liners.

Prepare the topping: In a small bowl, combine white sugar and softened butter. Mix with a fork until crumbly.

Set the topping aside.

Prepare the muffin batter: In a large bowl, combine brown sugar, oil, and egg.

In a separate bowl, combine buttermilk, baking soda, salt, and vanilla.

Alternately add the buttermilk mixture and flour into the brown sugar mixture.

Stir with a wooden spoon until just combined.

Gently fold in the diced apples and toasted pecans (if using).

Spoon batter into prepared muffin tins, filling each about 2/3 full.

Sprinkle the reserved topping mixture evenly over the muffins.

Bake for 30 minutes, or until a toothpick inserted into the center comes out clean.