Line a sheet pan with parchment paper and set it aside.
Melt the dark chocolate in a heatproof bowl set over a pan of barely simmering water, stirring often, until smooth and it reaches 46 C / 115 F.
Remove from heat and stir in half the orange zest.
Pour the melted chocolate onto the prepared pan and spread it into an even layer about 5mm thick with an offset spatula.
Scatter the candied orange peel, pistachios, remaining zest, and sea salt over the surface, pressing gently so they stick.
If using, drizzle the melted white chocolate over the top in thin lines.
Chill in the fridge for 60 minutes, or until fully firm and matte, not shiny or tacky to the touch.
Break into rough pieces by hand and store in an airtight container.