In a double boiler, melt the white chocolate (not the chips). You can use a microwave instead of a double boiler, but be very careful not to melt the chocolate too quickly and caramelise it.
Add the lemon curd and mix.
Add the cream and mix.
Put in the freezer to chill for 2-3 hours.
Take the mixture out of the freezer and shape into truffle-sized balls. Dust powdered sugar over your hands to stop the truffles melting around the edges. Put the balls on a baking tray covered with baking paper.
Put back in the freezer for 1 hour.
After about 30 mins, melt the white chocolate chips in a double boiler or microwave.
Let the melted chips sit for 20-30 mins to reach room temperature (this will give the truffles a thicker coating of chocolate). If you wait much longer, the chocolate will resolidify and need warming again- this is not a problem as the truffles can stay in the freezer for longer. If the chocolate has cooled too far, it will not coat the truffles evenly.
Put the melted chocolate in a small bowl or shotglass and coat each truffle with a layer of white chocolate. Put back on the baking paper and put in the fridge for a few minutes to set.