Place the chocolate in a bowl and if you're using something different than Valrhona break it into pieces.
Take the cream and put it in a pan, bring the cream to the boil and pour it over the chocolate, stirring to melt the chocolate.
When the chocolate is melted, add the butter and mix it into the ganache. Leave to cool while you temper your chocolate.
Take the tempered chocolate and pour it into your prepared moulds, tap them upside down and place any excess chocolate back in the bowl.
Using a scraper, scrape up and down the mould to ensure all excess chocolate is removed from the edges.
Place the mould in the fridge for 10 minutes to harden.
Put the ganache into a piping bag and fill the moulds two thirds full, so that you still have room to add chocolate on top.
Take more tempered chocolate and spread it over the top of the moulds, sealing up the truffles. Use the scraper and remove any excess chocolate so that the chocolates will come out of the moulds easily.
Place the chocolates back into the fridge for 10 minutes until hardened.
Put a towel on the worktop and tap out the chocolates onto the towel, if you bang the sides and the base of the moulds they should just fall out.
Take some of the ganache and pipe it onto the tops of the chocolates and place a crystallised violet on top for decoration.
For help with tempering chocolate look here. For help with the moulds look here