Place the white chocolate pieces and the cream into a glass or plastic bowl. Heat in the microwave on high for 1 minute at a time. Stir after each minute, until the chocolate has melted and the mixture is smooth.
Remove and stir in the butter and the liqueur.
Add the crushed ginger snap biscuits and stir in.
Chill until firm (at least 4 hours or overnight).
Once chilled and set, use a melon baller to scoop the mixture out.
Using clean hands, roll the mixture into nice, neat little truffle balls. Place the balls onto a tray lined with grease proof paper or cling film. Chill for about ½ hour while you melt the chocolate for coating.
Melt the baking chocolate: Bring a small pot half-filled with water to the boil on the stove.
Once the water boils, turn the heat off, but leave the pot on the stove.
Meanwhile, place the melting discs into a clean glass bowl, which fits snugly on top of the pot. Set the bowl over the hot water and leave until the chocolate has melted. Stir until smooth.
To dip the truffle balls: Pop a truffle ball into the melted chocolate and dunk to coat. Using a fork, lift the truffle out quickly and remove the excess dripping chocolate with a small sharp knife.
Lever the truffle off the fork and onto the greaseproof paper. Sprinkle with pistachios while the chocolate is still wet. Repeat with the remaining truffles and then chill to set.
Alternatively, roll the truffle balls in the crushed pistachios, without dipping in white chocolate first.