1. Break chocolate into squares and place in food processor. Grind until it resembles a coarse powder.
2. Heat the Irish cream, butter and Jameson 12YO until very hot (do not boil!). Pour slowly, while blending with the chocolate, into the food processor.
3. When smooth, add the creme fraiche and vanilla, if using, and blend again.
4. Leave mixture, covered, in the fridge for a couple of hours - it will get very thick.
5. Sift cocoa powder onto a large plate. Dust your hands with cocoa and roll small pieces (about a teaspoon) into a ball. You will need to work quickly so that the heat of your hands does not melt the chocolate.
6. Arrange in paper cases and refrigerate. The truffles will keep for about a week in the fridge (but won't last that long!)