Line a shallow baking tray with nonstick baking paper.
Heat the cacao, coconut oil and honey in a heavy-based saucepan over low-medium heat, whisking from time to time until the oil has melted and the mixture is silky smooth. Remove from the heat and pour the mixture onto the baking paper in a thin layer, about 5 mm thick.
Scatter over the pistachios and sea-salt flakes and place in the freezer for 20–30 minutes, or until set. Remove, break the bark into rough pieces and place in an airtight jar in the fridge. Lasts up to one month.