Combine chocolate, coffee and butter in a microwave safe bowl. Microwave on medium heat for 30 seconds, stir, then microwave for 30 seconds again. Stir constantly until all chocolate is melted and the mixture is smooth. Set aside to cool for 10 minutes.
Place cream in the bowl of your electric mixer. Beat until peaks form and you have whipped cream. I like to have firm peaks here and whip the cream to a thicker consistency as I find it makes the chocolate even lighter when folded in.
Add egg yolks, sugar and coffee to a saucepan over medium heat. Whisk constantly until sugar is dissolved, mixture is opaque and light yellow and has thickened and can coat the back of a spoon. Remove from heat and stir in melted chocolate mixture. Place saucepan in a bowl of ice water and whisk constantly until chocolate is cooled.
Remove whipped cream from the fridge and fold it into the chocolate mixture. Once whipped cream is completely folded in, evenly pour mousse into either two, four or six glasses depending on the size. This can serve six if served in 1/3 to 1/2 cup servings. Refrigerate for 4 hours. Top with fresh whipped cream, a sprinkle of cocoa powder and a coffee bean!