Preheat oven to 350°F and line two baking sheets with parchment paper.
In a large bowl, use a hand mixer with beaters to cream together softened butter and the sugars. Beat until totally combined and smooth, about 2 minutes.
Add peanut butter and vanilla extract and beat until combined. Add egg and beat until just combined.
Sprinkle the flour, baking soda, and salt over the mixture and use a spatula to fold everything together until combined and no streaks of flour remain. The dough will be soft and a bit sticky. Gently incorporate 3/4 cup of the chopped candy pieces with the spatula.
I used a 3 tablespoon scoop to scoop the dough into about 20 cookies. This makes smaller, about 3.25" cookies. For my Classic Peanut Butter cookies, I used a 1/3 cup scoop and baked the cookies longer (about 14-16 minutes) so that's an option if you want truly Crumbl-size cookies.
Bake cookies one sheet at a time until the cookies are very lightly crackled on top and the edges are turning golden, about 10-12 minutes. Don't overbake.
After removing cookies from oven, while they are still hot press into each cookie a sprinkle of Butterfinger pieces.
Cool on the cookie sheets for about 10 minutes and then transfer to a cooling rack to cool completely.
Once cookies are just slightly warm/cool, melt the chocolate chips in a small bowl in the microwave (use 10-20 second intervals and stir in between). Scoop into the corner of a small disposable sandwich bag and cut a tiny hole off the tip. Use this like a piping bag to drizzle chocolate over each cookie. This step is optional of course, but certainly adds some pizzazz!