How to Make It
Make the ganache In a small saucepan, bring the heavy cream and sugar just to a boil, stirring to dissolve the sugar. In a heatproof medium bowl, pour the hot cream over the chopped chocolate. Let stand for 2 minutes, then add the Armagnac and whisk until smooth. Press a piece of plastic directly onto the surface of the ganache and refrigerate until firm, at least 4 hours.
Scoop 2-teaspoon-size mounds of the ganache onto a parchment paper–lined baking sheet. Refrigerate the mounds until firm, about 30 minutes.
Using your hands, roll each mound of ganache into a ball. Refrigerate the balls until very firm, about 1 hour.
Make the coating In a microwave-safe medium bowl, heat the chocolate at high power in 20-second intervals, until nearly melted; stir between intervals. Let stand for 1 minute, then stir until smooth. Let cool slightly.
Put the cocoa powder in a medium bowl. Using a fork, dip each ganache ball in the melted chocolate, letting the excess drip back into the bowl, then coat in the cocoa powder and return to the baking sheet. Refrigerate the truffles for 2 hours before serving.