Preheat oven to 425 degrees Fahrenheit. Spray 12 cups of a muffin pan with non-stick cooking spray or line with paper muffin liners; set aside.
Crumb Topping
In a small bowl, add the flour, brown sugar, granulated sugar, and cinnamon.
Cut in the butter until the mixture is crumbly and no pieces of butter are larger than a pea.
Muffins
In a large mixing bowl, beat together the room temperature butter, brown sugar, and granulated sugar with an electric mixer on medium speed until smooth.
Add the eggs, sour cream, milk, and vanilla extract and mix until well combined.
In a medium mixing bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt.
Pour the dry mixture into the wet ingredients and mix together just until the dry ingredients are incorporated.
Fold in the chopped apple pieces.
Scoop the muffin batter into the cups of the prepared muffin tin, filling each to the top.
Spoon the crumb topping evenly over all of the muffins.
Bake the muffins at 425 degrees Fahrenheit for 5 minutes, then reduce the oven temperature to 350 degrees Fahrenheit and bake for an additional 15-18 minutes until a toothpick inserted into the center comes out clean.
Remove the muffins from the oven and cool in the pan for 5 minutes, then remove and cool completely on a wire cooling rack.
Notes
This recipe used Granny Smith Apples, but any firm baking apple will work. You could also use Honeycrisp, Gala, or Pink Lady Apples.The muffins keep well at room temperature or refrigerated for up to three days. They also freeze well for up to three months.The muffins should not be removed from the oven after the temperature is reset from 425 degrees Fahrenheit to 350 Fahrenheit. Simply reset the temperature and then set the timer for 15-18 minutes.