Line an 8×8-inch pan with foil, leaving about a 1-inch overhang, and lightly spray it. In a large bowl, combine the chocolates and sweetened condensed milk. Microwave on high for 3 minutes, pausing to stir after each minute. Once done, take it out and stir with a sturdy spoon until smooth.
Stir in the marshmallow fluff, vanilla, and butter until the mixture is smooth and fully blended. Spread the fudge evenly into the prepared pan, then gently tap the pan on the counter to level the top.
Chill, uncovered for 3-4 hours before cutting. Store in an airtight container.
