Creating the perfect Coffee Mousse involves following these clear steps:

1. Prepare Coffee: Brew a strong cup of coffee and let it cool to room temperature.
2. Melt Chocolate: In a microwave-safe bowl, combine the chopped dark chocolate. Microwave in 20-second intervals, stirring in between, until melted and smooth. Set aside to cool.
3. Whip Cream: In a mixing bowl, whip the heavy cream with 2 tablespoons of sugar until soft peaks form. Be careful not to overwhip. Gently fold in the cooled coffee and coffee liqueur until just combined. Set aside.
4. Beat Egg Whites: In a clean bowl, whisk the egg whites with the cream of tartar until foamy. Gradually add 1/4 cup of sugar and beat to stiff peaks.
5. Combine Mixtures: Gently fold one-third of the egg whites into the chocolate mixture to lighten it. Then, carefully fold in the remaining egg whites, taking care to maintain the lightness of the mixture.
6. Fold in Cream: Finally, fold in the whipped cream mixture until no streaks remain.
7. Chill: Spoon the mousse into individual serving dishes or a large serving bowl. Cover with plastic wrap and refrigerate for at least 2-4 hours or overnight, if possible.
8. Serve: When ready to serve, garnish with cocoa powder, chocolate shavings, or additional whipped cream.

Following these steps will allow you to create a decadent Coffee Mousse that will wow your guests and satisfy your taste buds.