Preheat your oven to 350 degrees F. Line a baking sheet with parchment paper. Set aside.
In the bowl of an electric mixer, fitted with the paddle attachment, combine the butter, peanut butter, brown sugar, and vanilla. Beat on medium speed until well blended, about one minute.
Add the egg and beat until just combined.
Add the flour, cocoa powder, baking soda and salt and mix on low speed until just combined.
Stir in the butterfinger candies and peanut butter chips.
Using a cookie scoop (mine is about 2 tablespoons), drop cookie dough on lined baking sheet about 2 inches apart. These cookies hold their domed shape quite well. If you want them flatter, use the back of a tumbler and bake them for 2 minutes less.
Bake for 10 to 12 minutes. Do not over bake. In fact, I like these underdone just a little. Let the cookies cool for about 10 minutes before eating.
Makes about 14 large cookies.