Line an 8×8-inch baking dish with parchment paper.
Chop the butterfinger candy bars and set aside ½ cup for topping.
In a medium saucepan over low heat, combine the chocolate chips, peanut butter chips, sweetened condensed milk, and butter.
Stir continuously until the mixture is smooth and the chocolate is fully melted. Remove from heat.
Stir in 1 cup of the chopped Butterfingers until evenly distributed.
Pour the mixture into the prepared pan and spread it evenly with a rubber spatula. Sprinkle the reserved ½ cup of Butterfinger pieces over the top, pressing them lightly into the fudge.
Refrigerate for about 1 hour, or until the fudge is firm and set.
Lift the fudge out of the pan using the parchment paper, then slice into 36 squares with a sharp knife. Enjoy!