Set up a double boiler on low heat. Add dark chocolate chips, butter, and sweetened condensed milk. Stir gently until smooth, about 5-7 minutes.

In a separate bowl, melt white chocolate chips with sweetened condensed milk in 30-second intervals until just melted. Add red gel food coloring and stir for a streaky effect.

Prepare an 8x8-inch pan with parchment paper. Pour ¾ of the chocolate mixture into the pan, add spoonfuls of the peppermint swirl mixture, then pour the remaining chocolate mixture over the top.

Using a skewer or butter knife, gently swirl the two mixtures together in wide figure-eights, ensuring distinct red and white swirls.

Sprinkle crushed peppermint candy on top before it sets. Refrigerate uncovered for at least 3 hours until firm and matte.

Once firm, lift the fudge out using parchment paper. Use a hot knife to cut into squares for serving.