In a medium saucepan, heat the cream over medium-high heat until bubbles appear around the edge. Remove from the heat and add chocolate.
Let stand for one minute and then stir until smooth.
In a small bowl, whisk the egg yolk, then whisk in 1 cup of the hot chocolate cream.
Transfer back to the saucepan, and whisk constantly until smooth.
Stir in rum and butter.
Pour into 6 ramekins, and refrigerate for 4 hours or until set.
Can be made ahead and refrigerated overnight.