Line an 8"x8" baking pan with parchment paper. Grease the parchment paper for easier release of the fudge.
Add chocolate and butter to a microwave safe bowl, and heat in 30 second intervals, stirring between each heat session, until fully melted.
Add marshmallow fluff, sweetened condensed milk, and vanilla. Stir well.
Add Butterfinger candy bits (reserving some aside for the topping), and stir to combine.
Pour into a baking pan, and use a spoon or knife to smooth out. Add remaining Butterfinger candy bits.
Cut into 5 sections, by 5 sections, to make 25 pieces.
Store in the refrigerator.