Preheat oven to 350°F. Line cookie sheets with silicone mats or parchment paper; set aside.
In a large mixing bowl, cream butter, brown sugar and white sugar with an electric mixer until light and fluffy.
Add eggs one at a time, beating until incorporated.
Beat in peanut butter, corn syrup and vanilla, mixing well. Stir in flour, baking soda and oats. Fold in M&Ms and chocolate chips, just until combined.
If dough is too sticky to handle, refrigerate for at least 30 minutes. Drop 3 tablespoons of dough on prepared cookie sheet, spacing cookies at least 2 inches apart. Press a few extra M&Ms on the tops for more colorful appearance, if desired. Bake at 350°F for 9-13 minutes.
Shorter time will result in softer, chewier cookies. Cookies will look slightly under-baked; allow to cool on cookie sheet for 5-10 minutes after removing from oven. Use the longer bake time if the dough has been chilled or crisp cookies are preferred. Cool on cookie sheet for 2 minutes, then remove to wire rack to cool completely.
Because these are called monster cookies, I like to make them extra large-sized – about 3 tablespoons or so. You'll get about 3 dozen with this size. The amount can be adjusted down to 1 1/2-2 tablespoons, which is about the size of a “regular” cookie and you'll get more cookies.