Making the Mousse:

1. Place the chocolate in a heatproof bowl. Set over but not touching barely simmering water in a saucepan. Heat, stirring, until the chocolate melts, about 2 minutes. Add the butter and continue to stir until the butter melts and is incorporated, 30 to 60 seconds.

2. Separate 1 egg, placing the white in a large bowl and adding the yolk to the chocolate. Quickly whisk in the yolk, fully incorporating it. Repeat with the remaining eggs. Remove the bowl from over the saucepan and let cool to lukewarm.

3. Add the salt to the egg whites. Using an electric mixer set on medium-high speed, beat the whites until they form stiff peaks, then beat in the confectioner's sugar. Using a rubber spatula, gently fold the egg whites into the chocolate, being careful not to deflate the whites.

4. Spoon the mixture into a large bowl or individual glasses. Cover and refrigerate until very firm, at least 6 hours or for up to 24 hours. Serve it well chilled.

If you like, garnish it with chocolate shavings.