Warm the cream and coffee in a small saucepan over medium-high heat, stirring frequently, until hot and bubbling around the edges, about 3 minutes. Remove the pan from the heat and let steep for 5 minutes.
Place the chocolates in a bowl. Strain the cream over the bowl of chocolate, discarding the coffee. Let sit for 30 seconds, then whisk until smooth. Spread the chocolate cream in a 9-inch pan or pie plate. Refrigerate until hard, at least 1 hour to overnight.
Place the cocoa powder on a plate or in a pie dish. Line a baking sheet with wax paper.
Using a measuring teaspoon, mini ice cream scoop or melon baler, scoop out 30 heaping spoonfuls of the truffle mixture. Place them in a single layer on the prepared baking sheet. When all of the scoops have been made, lightly roll them between the palms of your hands to give them a nice round shape. (If at any point the chocolate gets too warm and the truffles become difficult to roll, refrigerate the chocolate for 30 minutes until it firms up.)
Roll the truffles, a few at a time, in the cocoa powder. Place in a single layer on the second baking sheet. Cover with plastic wrap and refrigerate.
Before serving, let sit at room temperature to soften slightly, 15 to 30 minutes.
The truffles can be made a week in advance and kept refrigerated.
Makes 30 (1-inch) truffles
Adapted from "A Passion for Desserts" (Chronicle Books) 2003.