Begin by chopping the dark and white chocolate into small, even pieces for easy melting. Melt each type of chocolate separately in microwave-safe bowls, heating in 30-second intervals, stirring between each interval until smooth and fully melted. Alternatively, you can use a double boiler to melt the chocolate.
Line a baking sheet with parchment paper or a silicone baking mat. Pour the melted dark chocolate onto the prepared baking sheet and spread it out into a thin, even layer with a spatula. Repeat the same process for the white chocolate, pouring it over the dark chocolate layer and spreading evenly, creating a marbled effect if desired.
While the chocolate is still soft, immediately sprinkle the crushed candy canes, chopped nuts, dried cranberries, and mini marshmallows (if using) evenly over the chocolate. Press the toppings lightly into the chocolate so they adhere.
Place the baking sheet in the refrigerator for 1–2 hours, or until the chocolate is firm and set. Once set, break the bark into pieces by hand or using a sharp knife, depending on your preferred size.