Using an electric mixer, beat cream with vanilla until stiff peaks form. Cover and chill.

In a large metal bowl, combine egg yolks, butter, espresso, 2 tablespoons of the sugar, and salt. Set bowl over a saucepan of gently simmering water, making sure that the bowl does not touch the water. Cook, whisking constantly, until mixture is lighter in color and almost doubled in volume. The mixture will be very frothy. When an instant-read thermometer registers 160°, it is done. This should take between 1 and 5 minutes to achieve, depending on your heat and pan size, etc.

Remove bowl from saucepan. Add chopped chocolate and whisk until melted and smooth. Let stand, whisking occasionally, until mixture comes down to room temperature.

Using an electric mixer, beat egg whites in another medium bowl on medium speed until foamy. With mixer running, gradually beat in remaining 1 tablespoon of sugar. Increase speed to high and beat until firm peaks form.

Fold egg whites into chocolate in 2 additions. Fold whipped cream into mixture just to blend.

Divide dark chocolate mousse among six 6 to 8-ounce ramekins. Chill until firm, at least 2 hours. This dessert can easily be prepared one day in advance. Let stand at room temperature for 10 minutes before serving.

Right before serving, add a dollop of sweetened whipped cream, some chocolate shavings, and fresh strawberries.