Line a baking sheet with parchment paper.

Melt the dark chocolate in a heatproof bowl over barely simmering water, stirring until smooth, about 3 minutes.

Pour the dark chocolate onto the parchment and spread into an even 1/4-inch-thick rectangle with an offset spatula.

Melt the white chocolate the same way, then drizzle it over the dark chocolate in thick ribbons.

Drag a skewer or the tip of a knife through both chocolates in wide S-curves to marble them together.

While still wet, scatter dried cranberries, chopped pistachios, and gold sanding sugar over the surface, pressing lightly so they stick.

Refrigerate for 30 minutes, until the chocolate snaps cleanly when tapped.

Break into irregular shards and finish a few pieces with torn gold leaf before serving or packaging.