Place chopped chocolate in a medium-sized heatproof bowl and set aside.

In a small saucepan over medium heat, bring the cream to a boil. Once it begins to boil, remove cream from heat and whisk in the espresso powder.

Pour the cream over the chopped chocolate and allow the mixture to sit, untouched, for two minutes, then whisk smooth. Stir in the butter and vigorously whisk until smooth and shiny.

Cool to room temperature, then tightly cove the bowl with plastic wrap and refrigerate for 2 hours, or until firm enough to scoop.

Before you begin rolling your truffles, make sure you have enough room in your refrigerator for both baking sheets; the truffles must chill again before being dipped in the chocolate coating.