Separate 4 eggs into egg yolks and egg whites. Cover egg whites with plastic wrap and set aside.
Put egg yolks in a mixing bowl and, using a whisk, beat it very well for a few minutes until they become lighter in color and thicker consistency.
While you are whisking the egg yolks, put ¼ cup strong brewed coffee and ¾ cup granulated sugar in a sauce pan and heat it on the stove until all the sugar dissolves.
Gradually add the brewed coffee and sugar mixture into egg yolks while constantly whisking the egg yolks so they won’t cook.
Put a mixing bowl with egg yolks mixture over a pot of steaming water and constantly whisk the mixture for about 7-10 minutes until the mixture gets thick and pale color.
Put this thick and pale egg yolks mixture into the stand mixer bowl and beat it for 5 minutes on medium speed. It should become even thicker and more pale.
In a separate bowl whisk together 6 ounces melted chocolate and ½ cup softened butter until smooth consistency. Add melted chocolate mixture to the egg yolk mixture and fold it in.
In a separate bowl put egg whites, ¼ tsp salt and ½ tsp cream of tartar and, using a hand or stand mixer, beat it until you have stiff peaks formed.
Fold it together with egg yolk/chocolate mixture until smooth.
Divide the mixture between 4 serving glasses and refrigerate.
As an option, serve the chocolate mousse with a tablespoon of whipped cream.