Place a large heatproof bowl over a pan of simmering water. Add the dark chocolate and double cream, melting and mixing completely.
2. Step
Add the gin, tonic, lime juice, lime zest, and butter, and mix. Remove from heat and refrigerate for 3 hours, or overnight.
3. Step
Use a melon baller or spoon to scoop out the truffle mixture and shape into small balls. Freeze for 30 minutes.
4. Step
Using a fork, dip the frozen truffle balls into melted white chocolate to coat. Drizzle with more white chocolate, and sprinkle with lime zest. Refrigerate for 30 minutes, or until solid.
