Preheat the oven to 350°F. Line 2 baking sheets with parchment paper and set aside.
In a large bowl, cream together the butter and sugar until light and fluffy. Mix in the egg.
In a seperate bowl, mix together the flour, baking soda and salt.
Slowly mix in the dry ingredients into the wet ingredients until fully combined.
Fold in the butterfinger pieces.
Using a medium cookie scoop, scoop the cookie dough onto the prepared baking sheets, and bake in the preheated oven for 10-13 minutes. Remove from the oven and allow to cool completely on the baking sheets.