Mix the apple cider vinegar in with the milk and allow to sit until needed in the batter, or at least 3-5 minutes.
Mix together all dry ingredients for the muffins.
Make a well in the middle and add in the milk mixture and melted butter, stir well.
Fold in the diced apples.
Make the topping by stirring together the sugar, flour, and cinnamon. Cut in the butter until well incorporated, but don’t stir. Fold in the walnuts.
Fill the muffins liners ⅔ of the way up with the batter and then top with the topping mixture.
Bake for 20 minutes, or until a toothpick comes out clean.
(Optional) Make the icing while the muffins cool by stirring together the powdered sugar and milk, adding ¼ cup more powdered sugar at a time if needed or a teaspoon of milk to get your desired consistency. Then ice the muffins and enjoy.
Notes
These muffins will stay good at room temperature for 3-5 days.
It is not recommended to decrease the amount of sugar or spices in this recipe, if you do this the muffins will start to taste bland and won’t have a ‘pop’ of flavor like this recipe does.