How to Make It
Melt 1/2 of the chocolate in a double boiler over simmering water. Off the heat, whisk in the butter, then the egg yolks, espresso and rum; refrigerate until firm, about 1 hour.
Line a baking sheet with wax paper. Roll rounded teaspoons of the chocolate mixture into balls; place them on the baking sheet. Refrigerate for 1 hour.
Melt the remaining chocolate in the double boiler. Remove from the heat and let cool, about 10 minutes. Using a skewer, spear each truffette, dip it in the chocolate and return it to the baking sheet. Refrigerate until ready to serve.