Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.

In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.

In a large bowl, beat the softened butter, peanut butter, brown sugar, and granulated sugar together until light and fluffy (about 2-3 minutes).

Add the egg and vanilla extract to the butter mixture, beating until well combined.

Gradually add the dry flour mixture into the wet ingredients, mixing on low speed just until combined. Do not overmix.

Gently fold in the chopped Butterfingers and milk chocolate chips using a spatula.

Scoop about 2-3 tablespoons of dough per cookie and roll into balls. Place on the prepared baking sheets 2 inches apart.

Bake for 10-12 minutes until the edges are lightly golden but centers are still soft.

Let cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.