Prepare the tray: Line a flat baking tray with parchment paper and set aside.
Melt the chocolate: Prepare a double boiler by placing a heatproof mixing bowl over a saucepan of gently simmering water. Add the chocolate to the bowl and gently melt, stirring occasionally, until smooth. Alternatively, microwave the chocolate in a microwave-safe bowl in 20–30 second intervals, stirring well between each interval, until smooth and fully melted.
Add the peanut butter: Once the chocolate has completely melted, turn off the heat. Stir the peanut butter through the melted chocolate until smooth and fully combined.
Add the pistachios and sultanas: Add the pistachios and sultanas to the chocolate mixture. Stir gently until everything is evenly coated in chocolate.
Form the clusters: Using a teaspoon or small fork, scoop small bundles of the mixture onto the lined baking tray. Keep the mixture together as you form each cluster.
Use any leftover chocolate: If you have chocolate mixture remaining after making the clusters, add a few extra pistachios or sultanas and make additional clusters.
Sprinkle with sea salt: Place the clusters in the refrigerator for 5–10 minutes, just until the chocolate has started to firm slightly. Remove from the refrigerator and sprinkle a small amount of sea salt flakes over each cluster. Return to the refrigerator and chill for 1–2 hours, or until completely set.
Serve: Once set, remove the clusters from the refrigerator and enjoy.