Step 1: Put the white chocolate and cream into a medium heatproof bowl and melt over a pan of barely simmering water, stirring occasionally. Remove from heat and leave to cool, then stir in the shredded wheat, ground pistachios, and flaked sea salt. Cover and chill until firm, about 2 hours.
Step 2: Line a large baking sheet with baking parchment. Using a teaspoon or melon baller, scoop out slightly heaped portions of the chocolate mixture and roll into balls. Arrange on the lined tray and chill again until firm, about 1 hour.
Step 3: Melt milk chocolate in a medium heatproof bowl set over a pan of barely simmering water. Remove bowl from heat and stir until smooth. Leave to cool for 2 minutes.
Step 4: Working one at a time, drop a truffle into the melted chocolate, quickly roll to coat and lift out with a fork. Return to lined tray. Repeat until all the truffles are coated, then chill again until firm, about 2 hours. Serve.