To achieve the best results, use a high quality chocolate with 66% or 70% cocoa solids.
Prepare an ice bath in a large bowl and set aside.
Separate the eggs, placing the whites in the bowl of a stand mixer and the yolks in a medium heatproof mixing bowl. Take care when separating the eggs as even a small amount of yolk in the whites will prevent them from whipping properly.
To a medium saucepan set over medium heat, add 2 inches of water and bring to a simmer. To a medium heatproof bowl, add the chocolate, butter, and espresso powder and set the bowl over the simmering water; stir until the chocolate is melted and the mixture is smooth. Remove bowl from heat and set aside.
To the mixing bowl with the egg yolks, add 1/2 cup sugar and vanilla; whisk to combine. Set the bowl over the simmering water and whisk constantly until the mixture is thickened and warm. Continue to cook until it thickly coats the back of a spoon, about 4 minutes.
Remove the bowl from the simmering water and place the bowl into the prepared ice bath, stirring until the mixture is cool and thick. Remove from the ice bath and add the chocolate mixture into the egg yolk mixture, stirring with a silicone spatula until thoroughly combined. Refrigerate until ready to use.
To the bowl of a stand mixer fitted with a whip attachment, add egg whites with salt and whip until foamy. Gradually add the remaining 1 tablespoon of sugar and continue beating until medium peaks form. Fold one-third of the whites into the chocolate mixture until almost completely combined, and then fold in the remaining whites in 2 more stages. Stop folding when all of the whites are thoroughly incorporated, the mousse is smooth, and the color is even.
To serve: Spoon the mousse into individual 6-ounce serving dishes. Mousse can be served immediately or chilled up to 24 hours before serving. Top with whipped cream and chocolate curls.