In a medium bowl cream together the butter, brown sugar, and sugar with an electric mixer. Add the egg and then the vanilla and mix in after each addition.
Sift together the flour, oatmeal, baking soda, and salt add to the wet ingredients and mix until well combined. Add the peanuts and M&Ms and mix in with a wooden spoon or by hand.
Pat the cookie dough into the prepared skillet. Place in the preheated oven and bake 25 to 30 minutes or until the cookie is nicely browned.
It can be served right out of the oven (but be careful not to burn your mouth) or cooled and cut into wedges. If serving warm they are delicious with a scoop of vanilla ice cream.