I have made a similar recipe for years and it has never failed to become firm.
I was curious to see how the recipes differed, so I looked up the one I've used. Here it is.
Chocolate Mousse
6 oz. semisweet chocolate
2 tbs. Kahlua
1 tbs. orange juice
2 egg yolks
2 eggs
1 tsp. vanilla extract
1/4 c. sugar
1 c. heavy cream
Melt the chocolate in the Kahlua and orange juice over very low heat. Set aside. Put the egg yolks and eggs in a blender with vanilla and sugar. Blend for 2 minutes at medium high speed. Add the heavy cream and blend for another 30 seconds. Add the melted chocolate mixture and blend until smooth. Pour into a bowl or small individual cups. Refrigerate.
Makes 3 servings
Source: Annemarie's Personal Cook Book by Annemarie Huste
Although the recipe says that it makes 3 servings, it makes 4-6 pots de creme cups, and is very rich. You could certainly add espresso powder to give it a stronger mocha flavor.