Chop the chocolate finely and add it to a small microwave-safe bowl. Melt the chocolate in the microwave on 50% powder for 30-second increments until the chocolate is melted fully. Be sure to cool the chocolate after melting so it's almost to room temperature.
Add the room temperature egg yolks and melted chocolate to a blender and blend until smooth. Be sure the chocolate is cooled to just above room temperature and isn't hot when you add it to the blender.
With the blender running, pour the hot espresso into the egg yolk and chocolate mixture in a slow, steady stream and continue blending until the mixture is very smooth and silky. If you're adding alcohol, add it right after the coffee and blend well. Set the mixture aside.
Using a stand mixer fitted with the whisk attachment, or in a mixing bowl with a hand mixer, whip egg whites to hard peaks, adding the sugar slowly starting after the egg whites become foamy.
Fold the egg whites and the chocolate mixture together using a whisk or a spatula until no white streaks appear.
Pour into 4 individual serving bowls, ramekins, or a large shallow serving bowl.
Refrigerate for 1-2 hours to set, then serve.