Line an 8x8 inch square baking pan with parchment paper, leaving a 2-inch overhang on two opposite sides. This will help you lift the fudge out later. Lightly grease the parchment paper and any exposed sides of the pan.

In a medium, heavy-bottomed saucepan, whisk together the granulated sugar, cocoa powder, whole milk, and salt until combined. Place over medium heat.

Bring the mixture to a rolling boil, stirring constantly with a wooden spoon or heat-resistant spatula to prevent scorching. Once it boils, add the pieces of butter and allow them to melt.

Clip a candy thermometer to the side of the pan, making sure the tip is submerged in the mixture but not touching the bottom. Continue to cook, stirring occasionally, until the thermometer reaches exactly 234°F (112°C), which is the soft-ball stage. This should take about 8-10 minutes.

Once it reaches temperature, immediately remove the saucepan from the heat. Working quickly, add the marshmallow creme and vanilla extract. Stir vigorously until the marshmallow creme is completely melted and the mixture is smooth and glossy.

Pour the hot fudge into the prepared pan and use an offset spatula to spread it into an even layer.

In a small microwave-safe bowl, melt the white chocolate chips in 30-second intervals at 50% power, stirring between each interval until smooth.

Stir the peppermint extract and red food coloring (if using) into the melted white chocolate until you achieve a festive pink or red color.

Dollop the peppermint-white chocolate mixture over the top of the chocolate fudge. Use the tip of a knife or a skewer to gently swirl the colors together to create a marbled pattern. Be careful not to overmix.

Immediately sprinkle the crushed candy canes evenly over the top. Let the fudge cool on the counter for about 30 minutes before transferring it to the refrigerator.

Chill the fudge in the refrigerator for at least 4 hours, or until completely firm. For best results, chill overnight.

Once set, use the parchment paper overhangs to lift the entire block of fudge out of the pan. Place it on a cutting board and use a large, sharp knife to cut it into 36 small squares. Serve at room temperature.