Combine chopped chocolate and butter in a medium-sized microwave-safe bowl. Heat in 20-25 second intervals, stirring well in between, until ingredients are completely melted and mixture is smooth when stirred. If using espresso powder, stir that in here as well. Set aside.

In a separate large bowl, combine egg yolks, 1 Tablespoon of granulated sugar, salt, and vanilla extract. Whisk vigorously for about 30 seconds, until ingredients are lightened in color and slightly thickened.

Add the chocolate mixture into egg yolk mixture in 3 parts, whisking well after each addition until completely combined. Set aside.

Pour egg whites in another large, clean, dry, bowl, and use an electric mixer to beat until foamy. Slowly increase mixer speed to medium-high and add remaining sugar, about half a Tablespoon at a time (allowing about 10-15 seconds after each addition), until all sugar has been added and mixture reaches stiff peaks (see note if you have questions about stiff peaks).

Stir your chocolate mixture, then take a large spoonful (about ⅓-½ cup) of the egg white mixture and use a spatula to stir into the chocolate until completely combined to make it loose and easy to stir (the mixture should flow easily off the spatula). If the mixture is still thick, add a bit more egg whites.

Gradually, gently fold the remaining egg whites into the chocolate mixture. Do not over-mix and be as light-handed about this as possible. A few streaks remaining are fine.

In yet another bowl (yes, all of these dishes are worth it, I promise!) whip heavy cream on high-speed with an electric mixer until you reach stiff peaks.

Use a spatula to fold whipped cream into chocolate mousse mixture (again, being gentle and not over-mixing) until combined.

Immediately divide mousse into ramekins or serving dishes. Cover and refrigerate for at least 2-3 hours and up to 2 days before topping with whipped cream (instructions below) and serving.

Whipped CreamCombine heavy cream, powdered sugar, and vanilla extract in a large bowl. Use an electric mixer to beat until stiff peaks are just achieved.

Dollop over chocolate mousse before serving.