Preheat the oven to 350°F, and lightly oil a 10-inch cast iron skillet.
Add all of the ingredients except for the Peanut M&Ms to a mixer. Turn the mixer on medium speed, and beat until well combined and a dry dough forms.
Fold the Peanut M&Ms into the dough.
Turn the dough out into the cast iron skillet, and press it into the skillet with your hands until you get an even layer.
Place the skillet on the center rack of the oven, and bake 20 to 25 minutes, or until the edges of the cookie are golden-brown.
Allow the cookie to cool at least 10 minutes before serving with a large scoop of ice cream in the center and additional M&Ms. NOTE: The skillet will stay hot for a long period of time, so if serving to children, serve in separate bowls. Note: If serving individual slices of cookie, allow the skillet to cool to room temperature before serving, as the cookie will crumble when served hot.
Notes
*You can also use sweetened, salted creamy peanut butter. If using salted and sweetened, use only ¼ cup coconut sugar and omit the salt.
**You can replace the coconut sugar with ¾ cup tightly packed light brown sugar