1-In a large microwave-safe bowl, combine one bag of candy melts (either black or red) with half the sweetened condensed milk. Microwave on high for 1 minute, then stir, and continue in 30-second increments until fully melted and smooth. Stir in half the vanilla extract if using.

2-In a second microwave-safe bowl, repeat the process with the other bag of candy melts and the remaining sweetened condensed milk and vanilla extract.

3-Pour the melted black and red mixtures into the lined pan, alternating them in globs. Use a butter knife to create swirls by running it back and forth, side to side, and up and down through the mixture. Press in decorations like gummy fangs while the fudge is still wet if desired.

4-Refrigerate the pan until the fudge is set, about 3 hours.

5-Lift the fudge out of the pan using the parchment paper, trim the edges, and cut into 1- or 2-inch squares.