Preheat oven to 350 and have a rack in the middle. If you are doing cookies, line a baking sheet with parchment. If you are doing a skillet cookie, don't do anything to your cast iron.
Beat the butter (or shortening), sugar, egg, milk and vanilla extract together with a paddle attachment in a stand mixer (or use a hand mixer). In a separate bowl whisk together the flour, baking soda and salt. Add the flour blend to the butter blend and mix together. Stir in the granola cereal.
For cookies drop by tablespoon onto the lined cookie sheet. For the skillet cookie, spread batter in the cast iron.
Bake the cookies for 12 minutes. Bake the skillet cast iron cookie for 20 minutes. Cool before enjoying. If you're really nice, serve the skillet cookie drizzled with caramel and maybe even some ice cream :)