Line an 8-inch square pan with parchment, leaving overhang; lightly grease.
Warm a medium saucepan over low heat. Add chocolate chips, condensed milk, and butter.
Stir constantly until smooth and glossy, 5–7 minutes. Remove from heat.
Stir in salt and vanilla. Fold in about half the chopped candy bars.
Spread the fudge into the pan. Sprinkle on remaining candy and gently press to adhere.
Chill until firm, at least 2 hours. Lift out, slice into small squares, and serve.