Step 1 Place the chocolate in the bowl of a food processor fitted with the steel blade and pulse to chop the chocolate very fine.
Step 2 In a small saucepan, combine the espresso, Grand Marnier and 1 tablespoon of the cream and bring to a boil. Remove from heat. Turn on the food processor and add the hot liquid with the processor running. Process until the chocolate has melted. Stop the processor and scrape down the sides. Continue to process until smooth. Add the egg yolk and process until blended. Scrape into a large bowl.
Step 3In the bowl of a stand mixer fitted with the whisk attachment (or in a large bowl, with a hand mixer), beat the egg whites on medium speed. When they foam, add the cream of tartar and continue to beat on medium speed while you slowly stream in the sugar. Beat until the egg whites form medium peaks. Fold into the chocolate base, in two additions.
Step 4Beat the remaining cream until it forms soft peaks. Fold into the mousse. Spoon into 6 (4-ounce) ramekins or other serving cups and refrigerate for 2 hours, or until set. Serve with whipped cream and chocolate shavings, if desired.