Preheat oven to 350ºF and line a cookie sheet with parchment paper.
Chop candy into small pieces if necessary.
In a stand mixer fitted with the paddle attachment, cream together the brown sugar, butter, and peanut butter until light and fluffy, about 2-3 minutes.
With the mixer on the lowest speed, add in the egg.
In a separate bowl, add the flour, baking soda, cornstarch, baking powder, and salt. Sift to remove lumps.
Slowly add the flour mixture to the wet ingredients and mix until just combined.
Stir in the oats and all candies by hand.
Use a medium ice cream scoop (2-3 tablespoons) and scoop out cookie dough balls. Place about 6 on each baking sheet.
Bake the cookies for 8-10 minutes until the edges are just starting to brown and the top still looks slightly undercooked.
Remove from the oven and cool for 5 minutes on the baking sheet before moving them to a wire rack to cool completely.