Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the butter, brown sugar, and sugar. Cream together until light and fluffy, about 3-5 minutes.
Add the eggs one at a time, mixing well after each addition. Then, add the vanilla extract and mix until combined.
Add the peanut butter and beat until fully incorporated and smooth.
In a separate bowl, whisk together the baking soda and oats. Gradually add to the wet ingredients, mixing until just combined.
Fold in the chocolate chips, chopped walnuts, and M&Ms gently.
Scoop heaping amounts of dough onto prepared baking sheets, leaving space for spreading.
Bake for 10-12 minutes, or until edges are lightly golden.
Cool on the baking sheet for 5 minutes, then transfer to a wire rack. Garnish with extra M&Ms if desired.
