Melt the chocolates together in a bowl over a pan of simmering water (bain-marie) or in the microwave.
Mix the tub of clotted cream into the chocolate mix and chill in the fridge for at least 4 hours, or even over night.
Once chilled, use a melon baller or small scoop to create truffle balls or shapes and roll each of them in the sifted cocoa powder until completely covered
Chill in the fridge again for a few more hours as the mixture would have softened considerably after rolling and then serve or package cutely as a present!
Notes
I find it easier to shape my truffles with a melon baller as I find them the perfect size truffles - if you don't have one however you could try using two teaspoons to get the right amount - and then rolling them in your hands to create a ball shape.