Over low heat in a small heavy saucepan, melt the chocolate with the espresso.

While chocolate is melting, cut the butter into small pieces and put it in the bowl of a food processor fitted with a metal blade. Process the butter with the sugar until fluffy, about 1 minute.

Add warm melted chocolate-espresso mixture. Process 15 seconds. Scrape sides of bowl and process for another 20 seconds.

Add eggs, one at a time, and process for 15 seconds with each egg addition.

Divide the mousse into individual serving dishes and refrigerate until well chilled, at least 4 hours.