Preheat the oven to 350°F and line 2 sheet pans with parchment paper.

Whisk together the flour, oats, baking soda, and salt in a medium mixing bowl.

In a large mixing bowl, cream the brown sugar and butter with an electric mixer until light and fluffy, about 2 to 3 minutes. Add the peanut butter, eggs, and vanilla extract, then mix until combined.

Slowly add the flour mixture to the mixing bowl and mix just until the dough forms.

Add a few drops of purple gel food coloring and mix until evenly distributed. Continue adding a few drops at a time until the desired color is achieved.

Reserve about 1/4 of the Halloween M&M’s for topping. Add the remaining M&M’s and the chocolate chips to the dough and mix just until incorporated.

Using a 3-tablespoon-size cookie scoop, scoop out balls of dough and place them onto a sheet pan. Place the dough in the refrigerator for at least 15 minutes to chill.

Place the cookies on the prepared sheet pans about 2 to 3 inches apart. Press a few of the reserved Halloween M&M’s into the top of each cookie.

Bake the cookies in the preheated oven for 15 to 16 minutes, or until the bottoms are golden brown.

As soon as the cookies come out of the oven, gently press the candy eyes into the warm cookies.

Allow the cookies to rest on the sheet pans for a couple of minutes before transferring them to a wire cooling rack to cool.