Whip the heavy cream with a stand mixer or hand-held mixer on high until it has soft peaks, then place in the fridge to chill.
Put the chocolate in the bowl of a double boiler, along with the butter and espresso. Stir constantly until the chocolate has melted and everything is incorporated. Be careful not to overheat, or the chocolate will seize. Remove from heat.
While the chocolate is cooling, whip the egg whites until they start to get foamy. Add the sugar and continue to whip the egg whites until they hold stiff peaks.
Once the chocolate is warm to the touch, add the egg yolks and stir until fully combined.
Remove the whipped cream from the fridge and stir in roughly 1/3 of the whipped cream into the chocolate mix, gently, until fully incorporated. This will help loosen up the chocolate mix and make it runnier. Next, fold in ~1/2 of the egg whites, very gently. Once incorporated, fold in the remaining egg whites, followed by the rest of the whipped cream.
Once everything is smooth and evenly mixed, you can either spoon or pipe it into your desired serving glasses or dishes. Chill the mousse in the fridge for at least 8, or up to 24 hours. Top with fresh whipped cream (for a full whipped cream recipe check this out!) and/or fresh fruit as desired!