Combine flour, baking soda and salt in a small bowl and whisk to combine.
Combine butter and sugar in a stand mixer fitted with the paddle and mix on medium speed until creamy, scraping down sides of bowl at least once.
Add the egg and mix well.
Add flour mixture and mix until well combined.
Stir in Butterfinger pieces with a wooden spoon.
Drop mixture with a 1½ tablespoon scoop onto prepared baking sheets (alternately, drop by slightly rounded tablespoons).
Six cookies to a large baking sheet.
Bake 10-11 minutes or until just beginning to brown, cookies should look slightly underbaked. Remove from oven and allow to cool on baking sheet 2-3 minutes before removing to a rack to cool completely.