Start by preheating your oven to 350°F (175°C). This ensures that your cookies bake evenly and become perfectly golden.
In a large mixing bowl, combine the creamy peanut butter, large egg, brown sugar, granulated sugar, vanilla extract, and nonfat milk. Use a spatula or a hand mixer to blend these ingredients until smooth and creamy.
Sprinkle the baking soda over the mixture and stir well. Then, gradually fold in the gluten-free oats until they are evenly distributed throughout the batter.
Gently fold in the M&Ms (or your choice of mix-ins) until they are well incorporated into the dough.
Using a tablespoon or cookie scoop, drop rounded balls of dough onto a baking sheet lined with parchment paper. Make sure to leave some space between each cookie, as they will spread slightly during baking.
Place the baking sheet in the preheated oven and bake for 10-12 minutes, or until the edges are golden brown. The centers may look slightly underbaked, but they will set as they cool.
Once baked, remove the cookies from the oven and let them cool on the baking sheet for about 5 minutes. Then, transfer them to a wire rack to cool completely. Enjoy your delicious Gluten Free Monster Cookies with a glass of milk or your favorite beverage!