Preheat the oven to 375°F. Line a baking sheet with parchment paper (this is a must, candy can be sticky!).
If needed: Place the candy bars into an airtight plastic bag and using a mallet, crush into coarse pieces, if not already crushed.
In the bowl of your stand mixer fitted with a paddle attachment or large mixing bowl, cream together the butter, white sugar and dark brown until light and fluffy.
Blend in the eggs and vanilla.
In another medium bowl, sift together the flour, corn starch, baking powder and salt. Add half of the flour mixture, blend and then add the remainder. The dough will be thick and dry. Finally, fold in the crushed candy bar.
Use a 1 1/2 tablespoon cookie scoop and place 2 inches apart on the prepared baking sheet.
Bake for 11-12 minutes. Cookies will look semi-cooked, this is the time to take them out! If you bake them too long, they won't be chewy. Remove and move to a wire cooling rack.
If you've tried this recipe, come back and let us know how they were in the comments or star ratings.
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Notes
You can now buy crushed Butterfinger pieces in the baking aisle!