In a saucepan slowly bring the cream to the boil. Turn off the heat and add the coffee and chocolate. Stir with a whisk until the mixture becomes smooth and the chocolate has melted. The liquid should have become slightly lighter and shinier at this stage. Pour into a jug and leave to cool for about 2 hours or until it has come down to room temperature. See Cook's note*.
Beat the mixture with an electric whisk for about 1 minute or until it changes color.
Line a square plastic container with baking paper and pour the chocolate mixture into it. Leave to set in the refrigerator for 2 hours. You can test this by pressing the top with your fingers. Run a spatula around the edges and lift the chocolate on the baking paper out of the dish and swiftly cut it lengthways into even strips of about 2/3-inch. Then cut these into cubes. Put cocoa powder into a flat dish and roll the cubes in it, making sure that they are well coated. The truffles can be kept for about 2 weeks.
Alternatively you can place it in the refrigerator for 20 minutes, giving it a stir every 10 minutes.