Place the chocolate in a large bowl. Bring the cream to the boil and pour it over the chocolate. Stir gently until chocolate has melted, trying not to create bubbles; leave to cool for 2 minutes.
Add the butter in 2 stages and stir in gently. Once the butter is incorporated the ganache should be smooth and glassy with no oil slick on the surface. If the mixture splits and an oil slick appears put the whole lot in a food processor and blend briefly until the mixture re-emulsifies.
Set the truffle mixture in the fridge for a minimum of 3 hours. For best result leave for overnight.
Remove the ganache from the fridge about 15 minutes before you want to make the truffles depending on room temperature. Put the cocoa into a bowl.
Ensure your hands are cold and dry then dust them with cocoa. Take spoonfuls of the ganache mixture and roll the mixture into a ball in your cocoa-dusted hands.
Drop each shaped truffle into the bowl of cocoa, turn it around and then toss it between your palms to remove any excess powder.
The truffles can then be returned to the fridge and kept for up to 2 days as long as they are stored in an air tight container