Choose some apples that are a little tart. You can't go wrong as long as they're crisp and flavorful.
Wash and core apples with a cheap but effective APPLE CORER. It works a lot faster than a paring knife.
Don't quite pierce the bottom of the apple; just drill down to about the last 1/2" -1/4". Your opening needs to be almost 1" wide.
Place in baking dish with sides. Fill the centers of each apple with 1-2 tablespoons of brown sugar, a teaspoon of walnuts or pecans, 1 tsp. of butter, and a sprinkle of cinnamon. I actually don't measure anything in this recipe myself, I just use my fingers and approximate!
I've given you the basic ingredients... but you could substitute or add anything you think tastes good with apples.
For the final touch, take a marshmallow and rudely stuff it in the top of each apple, pushing down the other ingredients into the cored hole.