Preheat the oven to 375°F. Generously grease or line with muffin cups a 12-cup muffin tin.
In a medium bowl, toss apples, 2 teaspoons flour, and 1/2 teaspoon cinnamon.
In a large bowl, whisk together the oil, apple sauce, and sugar until combined. Beat in eggs, vanilla, and buttermilk until well combined.
In a small bowl, whisk together the remaining 2 cups flour, baking powder, salt, apple pie spice and remaining cinnamon. Stir flour mixture into wet ingredients. Fold in the apples.
Divide batter evenly between the tin cups. Bake 30 minutes, or until a toothpick inserted in the center comes out clean and the edges are golden. Remove pan from oven and place on a cooling rack.
Topping
In a small bowl, stir together the sugar and cinnamon. Brush the tops of the muffins with butter, then roll in the cinnamon-sugar mixture. Serve warm.
Notes
Apples: While I prefer granny smith, honey crisp also works great. If you prefer a softer apple, you can grate the apples instead of having bigger pieces. Mini muffins: You can also bake in a mini muffin pan (makes about 48 mini muffins). For mini muffins bake about 16 to 18 minutes.Apple pie spice: If you don't have apple spice you can use 1 1/4 teaspoon cinnamon, 1/2 teaspoon nutmeg and 1/4 teaspoon all-spice.Storage:Best served warm the first day. Store leftovers in a covered container up to 3 days. Freeze up to 3 months.Recipe updated 8/22