Preheat oven to 350 degrees. Line two cookie sheets with parchment paper.
In a large bowl (or the bowl of a stand mixer) cream together the butter, granulated sugar, brown sugar, and peanut butter until light and fluffy, about 2-3 minutes
Mix in the egg and vanilla, scraping down the sides as necessary.
In a separate medium size bowl, whisk the oats, flour, baking soda, cornstarch, baking powder and salt.
With your mixer on low speed, add the dry ingredients to the wet in two additions.
Stir in chopped Halloween candy and Smarties.
Place dough in the refrigerator for 10 minutes. (This step will make creating the balls easier if you’re using your hands. If your dough gets too warm, it will also keep the cookies from spreading during baking.)
Using an ice cream scoop or your hands, form golf ball-size rounds and place on prepared baking sheets.
Bake cookies for 7-9 minutes or until edges start to brown but middle looks slightly under-baked. Allow cookies to cool for 5 minutes before transferring to wire rack to cool completely.