2mediumapples, peeled and cut in small dice (about 2 cups)
Preheat oven to 350F. Butter a 12-cup muffin tin, or line with muffin liners.
Mix up the crisp topping using your fingers. Rub the butter into the dry ingredients until there is no dry flour left and the mixture has a fine crumbly texture.
In a large bowl, whisk together the flour, brown sugar, apple pie spice, baking powder, baking soda and salt until everything is fully incorporated. Make sure to blend in any lumps of brown sugar.
In a separate bowl, whisk the eggs, vegetable oil, buttermilk, and vanilla extract until smooth.
Add the wet ingredients to the dry ingredients, along with the chopped apples, and fold together gently until there are no dry bits of flour left. Do not over mix at this point.
Fill your muffin cups quite full, and then sprinkle generously with crisp topping. Note: At this point you can bake right away or let the muffins sit on the counter for an hour. I find this results in a higher rise, but it's optional. If you choose to let the muffins rest for an hour, wait to add the crisp topping untl just before baking.
Bake for about 25 minutes until risen and golden. You can check with a toothpick to see if the center is done if you're unsure.
Let cool 10 minutes, then remove the muffins to a cooling rack. Enjoy while still warm with a little butter, or rewarm in the microwave for 30 seconds.