Preheat oven to 350° F. Lightly butter a 12'' x 17'' jelly roll pan with 1 tablespoon of the butter. Set aside.
In the bowl of a stand mixer, cream together remaining ½ cup butter, peanut butter, brown sugar, and sugar. Add eggs and vanilla, and mix until incorporated. Add flour, oats, baking soda, and salt, and mix again until incorporated.
Reserve about ½ cup each of the chocolate chips and M&M's, and then fold the remaining into the batter with a spatula. Press the batter evenly into prepared pan. Sprinkle the reserved chocolate chips and M&M's over the top, pressing them in slightly. Bake for 14 to 15 minutes. These bars are meant to be chewy; do not over bake! Remove from oven and let pan cool completely on wire rack. Then cut into individual bars.
