Preheat your oven to 350°F (175°C).
Line a 9x13 inch baking pan with parchment paper.
Cream together the softened unsalted butter and peanut butter until smooth and fluffy (about 2-3 minutes).
Add the packed brown sugar, large egg, vanilla extract, and milk. Mix until well combined.
In a separate bowl, whisk together the all-purpose flour, baking soda, and salt. Gradually add to the wet ingredients, mixing until just combined.
Fold in the peanut butter chips, chopped Butterfinger pieces, and chocolate chips until evenly distributed.
Spread the batter in the prepared baking pan. Bake for 25-30 minutes, or until the edges are lightly golden.
Allow to cool for 10-15 minutes, then lift out using parchment paper and let cool completely before slicing.