Line a baking sheet with tin foil.

Toss pistachios, cranberries, and orange zest in a bowl.

Melt 18 oz chocolate in a double boiler over hot water (You can also microwave the chocolate in 30-second increments instead). Stir often with a spatula so it melts evenly.

Remove chocolate from heat, then stir in remaining 6 oz chocolate, mixing until thoroughly melted and smooth.

Add 1/2 of the pistachio mixture to the chocolate and stir well.

Quickly scrape the chocolate onto the baking sheet, using the spatula to spread it to an even 1/4-inch thickness.

Sprinkle the remaining pistachio mixture on top, gently pressing it into the chocolate.

Refrigerate uncovered just until set, 30-45 minutes.

Flip the baking sheet over onto a large cutting board. Remove the baking sheet and peel the tin foil off. Using a knife, cut into 2 to 3-inch chunks of bark.