Preheat oven to 350 degrees F. As it heats, beat the butter and sugars in a large bowl until light and fluffy, about 2 minutes. Blend in the egg and vanilla.
In another bowl, combine the flour, cocoa, baking soda and salt, then gradually stir in the butter/sugar mixture and the oats. Once those are all mixed together, fold in the peanut butter chips and crushed M&Ms.
Place one-inch blobs of dough on a baking sheet about 1 1/2 inches apart. Bake for 9 to 11 minutes, or until edges have set and the center still appears moist. (It will finish cooking as it cools.) Once cool, transfer to an airtight container, where they'll keep for up to a week.
