In a saucepan, combine the creme fraiche and espresso and bring to a boil over medium heat. As soon as it boils, turn off the heat. Meanwhile, put the chopped bittersweet chocolate in a medium bowl. Strain the hot creme fraiche mixture into the bowl. Whisk until the chocolate is melted and the mixture is smooth. Cover and let rest in a cool place overnight. The mixture will become firm but not too hard to pipe easily.
The next day, using a pastry bag fitted with a large plain tip, pipe bite-size "kisses" of the mixture onto parchment paper-lined baking sheets. Refrigerate briefly just until set, about 30 minutes. Use your palm to gently press down the point that sticks up on each truffle. Transfer to the freezer and freeze until hard, 2 to 3 hours or overnight.
In the top of a double boiler set over barely simmering water, melt the semisweet chocolate. It should be liquid, but not so hot that you can't touch it; if it is too hot for you, wear disposable surgical gloves. Spread the cocoa powder out on a sheet pan with sides. Working in 2 batches if necessary to avoid crowding the pan of cocoa, dip the frozen truffle centers 1 at a time into the melted chocolate, shake off any excess, and set them down in the cocoa. When all the truffle centers are dipped and the chocolate has started to set, gently but thoroughly shake the sheet pan to roll the truffles around in the cocoa until coated. Carefully remove to another sheet pan and refrigerate, uncovered, 30 minutes. You can sift the unused cocoa and use it for another purpose. Transfer to an airtight container and keep chilled until almost ready to serve. Bring to room temperature before serving.
Courtesy of Gale Gand, "Butter Sugar Flour Eggs" by Gale Gand, Rick Tramonto, Julia Moskin, Clarkson N. Potter Publishers, 1999