In a large heatproof bowl, add chopped chocolate and butter. Melt them in the microwave in 30-second intervals, stirring after each increment. When most of the chocolate has melted, remove it from the microwave. Stir it until the rest of the chocolate has melted. Let the melted chocolate cool down for a minute. 160 g semi-sweet chocolate, 1 Tablespoon butter
Alternatively, melt the chocolate in a double boiler. Place the bowl of chocolate and butter over a saucepan with an inch of simmering water. Stir until the chocolate melts.
Separate the egg yolks from the whites. Add the whites to a medium bowl. 3 large eggs*
In a small bowl or glass, dissolve the instant coffee powder in the hot water. 2 1/2 Tablespoons hot water, 1 1/2 Tablespoons instant espresso powder
Add the egg yolks to the chocolate mixture, one at a time. Mix in each egg yolk well before adding the next. Stir in the coffee mixture and salt. Set this aside. 1/4 tsp salt
Whip the egg whites in a bowl on high speed until foamy. Stream in the granulated sugar and keep whipping until stiff peaks form and the sugar has dissolved. 50 g granulated sugar
Fold the egg white mixture into the chocolate mixture and gently fold it with a rubber spatula. It should be all incorporated but don't over-mix it, or you will lose the airiness.
Add the heavy cream to a separate bowl and use an electric mixer to whip the cream on medium-high speed until stiff peaks hold. 120 g heavy cream
Add the cream to the chocolate mixture and fold it gently until it is all incorporated. Spoon the mixture into serving glasses. Chill for at least 6 hours or overnight.