Place the semi-sweet chocolate in a heatproof bowl.
Over medium heat, whisk the espresso powder into the cream and warm the cream until just before it boils. Pour the cream over the chocolate and mix together until the residual heat from the cream has melted the chocolate. Place this mixture in the refrigerator until it hardens (45–50 minutes or overnight).
Once the chocolate mixture has set, use a melon baller or a spoon to scoop out teaspoons of the chocolate mixture. Roll these into balls and place on a piece of parchment paper on a cookie sheet.
Place the dark chocolate in a saucepan set over a pan of simmering water. Melt the chocolate and stir until smooth. Dip the truffle balls one by one into the dark chocolate and place them back on the cookie sheet. Sprinkle half of the batch with the fleur de sel.
When the chocolate coating has hardened, roll the remaining truffles in the cocoa powder.
Store all of the truffles in the refrigerator.
Adapted from a recipe by Nancy Swartzendruber.