Preheat the oven to 350 degrees Line a large baking sheet with parchment paper or a Silpat baking mat and set aside.
In a medium bowl, whisk together the flour, cocoa, baking soda, and salt. Set aside.
Using a stand mixer or hand-held mixer, beat the butter and sugars together until creamy and smooth. Scrape down the sides of the bowl with a spatula, if necessary. Add the peanut butter and beat until combined.
Add in the egg and vanilla extract. Mix until well combined.
Add the dry ingredients and mix on low until just until the combined. Don't over mix. Gently stir in the oats, chocolate chips and M&M’s.
Scoop the cookies into round balls, about 2 tablespoons of dough per cookie, and place on the prepared baking sheets, leaving 2 inches in between cookies. Bake for 10-12 minutes, or until the cookies are lightly browned around the edges, but still soft in the middle.
Remove cookies from oven and sprinkle the cookies with sea salt. You can also gently press additional chocolate chips and M&M’s on top of the cookies to make them look extra pretty. For Halloween, you can even add candy eyeballs.
Let the cookies cool on the baking sheet for about 5 minutes or until set. When set, transfer cookies to a wire rack and cool completely.
Store the cooled cookies in a sealed container on the countertop for up to 3 days.