Preheat oven to 375F (190C) and line a 12-count muffin tin with paper liners. Set aside.
In a large mixing bowl, whisk together flour, sugars, baking powder, cinnamon, and salt.
In a separate bowl (or large measuring cup), whisk together oil, buttermilk, eggs, and vanilla extract until well-combined.
Add the wet ingredients to the dry ingredients and use a spatula or spoon to gently stir ingredients together until approximately 50-75% combined. Add apple pieces and continue to stir until ingredients are just combined (over-mixing may result in dense, dry muffins).
Evenly divide batter into prepared muffin tin. Set aside while you prepare your crumb topping.
In a medium-sized bowl, whisk together flour, sugars, cinnamon, and salt.
Drizzle melted butter over the flour mixture and use a fork to toss/claw the mixture together until all of the dry ingredients are moistened and you have a crumbly streusel.
Evenly divide streusel over the muffins.
Transfer to center rack of 375F (190C) oven and bake for 24-25 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Allow muffins to cool in muffin tin for 10 minutes before carefully removing to a cooling rack (I use a butter knife to gently pry them up and then lift them from the pan) and allow to cool a bit before enjoying.
