Separate 4 eggs into egg yolks and egg whites. Cover egg whites with plastic wrap and set aside.
This step is very important! 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.
NOTE: By whisking the egg yolks before adding hot ingredients to it, will prevent the eggs from getting “cooked” and lumpy.
While you are whisking the egg yolks, put strong brewed coffee and sugar in a sauce pan and heat it on the stove until all the sugar dissolves. You don’t need to boil it, just dissolve the sugar.
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 10 minutes until the mixture gets thick and pale color.
NOTE 1: For the best results, bring the water to boil first, then reduce the heat to low. We don’t need the water to boil, but only to release hot steam. After you lowered the heat, place the bowl with egg yolks on top and whisk.
NOTE 2: The whisking time over hot water can take different time (up to 20 minutes). As the final result, we want the mixture to get thicker consistency, until the ribbons form.
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 paler.
In a separate bowl whisk together melted chocolate and 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, salt and 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.
Scoop the mousse in the chocolate graham crust and refrigerate overnight.