Put the butter, syrup and cocoa powder in a pan over a medium heat, stirring until the butter has melted. Remove from the heat and stir in the milk chocolate until melted. Add the biscuits, dried fruit and nuts, then stir until everything is well combined and evenly coated in the chocolate mixture.
Pour the biscuit mixture into the prepared tin, then press into an even layer using the back of a spoon. Chill for at least 1 hour or until firm.
For the topping, melt the milk chocolate, either in a bowl set over a pot of barely simmering water (don’t let the bowl touch the water) or in the microwave in short bursts of 10 seconds at a time. Pour the melted chocolate over the biscuit mixture, tipping the tin so the chocolate forms an even layer. Return to the fridge for 20 minutes to set.
Remove the tiffin from the tin. Melt the white chocolate (in a bowl as described above – don’t stir), then drizzle over the top of the tiffin. Decorate with the 25g chopped pistachios and, if you like, edible glitter and/or edible sprinkles. Cut into small squares to serve.
