Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper. Chop the Butterfinger bars into small pieces and set aside.
In a large bowl, beat together the peanut butter, softened butter, granulated sugar, and brown sugar on medium speed for 2–3 minutes until light and fluffy.
Mix in the egg, milk, and vanilla extract until smooth and creamy.
In a separate bowl, whisk together flour, baking soda, and salt. Gradually add to the wet mixture, mixing just until combined. Do not overmix.
Fold in the chopped Butterfinger candy pieces until evenly distributed throughout the dough.
Scoop tablespoon-sized portions of dough and place them on prepared baking sheets, spacing about 2 inches apart.
Bake for 10–12 minutes, or until the edges are lightly golden but the centers remain soft.
Cool cookies on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
