01 -
Lightly grease an 8x8-inch pan and line it with parchment paper, leaving overhang on two sides for lifting out the fudge.
02 -
Place red candy melts and black candy melts in separate microwave-safe bowls. Microwave one bowl at a time in 30-second bursts, stirring after each burst, until smooth.
03 -
Remove the lid and foil from the frosting container. Microwave the frosting for 35-45 seconds, until loose and pourable. Stir in the salt and vanilla if using.
04 -
Divide the warm frosting evenly between the two bowls of melted candy. Stir each bowl quickly until the fudge mixture is smooth, glossy, and evenly colored.
05 -
Spoon alternating dollops of red and black fudge into the prepared pan. Drag a skewer or thin knife through the mixture a few times to create bold swirls, then tap the pan gently on the counter.
06 -
Cover and refrigerate for at least 4 hours, or overnight, until the fudge is firm enough to lift from the pan.
07 -
Lift the fudge out using the parchment. Cut into 25 small squares with a sharp knife, wiping the blade between cuts for clean edges.