Melt chocolate in a double boiler or heat proof bowl over water on medium heat.
Remove the pot or bowl with the melted chocolate from the heat. Mix in the sugar, coffee, and egg yolk until smooth. Remove from heat and let cool a bit, until lukewarm. (You don't want it so hot that it melts the whipped cream, but you also don't want it so cool that it starts to harden.)
Meanwhile, whip the heavy cream until stiff peaks start to form.
Use a spatula to fold the chocolate mixture into the whipped cream. Combine until just mixed--you don't want to overmix or it will be too dense.
Dollop the mousse into 6 little coffee cups, glasses, or bowls.
Chill in the refrigerator for one hour. (They will keep for up to 3 days if tightly covered).
Remove from the refrigerator 10-15 minutes before serving. Serve with desired toppings and enjoy!
