Melt the chocolate in a medium heatproof bowl placed over a small pot of simmering water. Stir until the chocolate is melted and smooth.
Remove bowl from water, add the egg yolks, and whisk until smooth.
In the bowl of a stand mixer fitted with a whisk attachment or in a bowl with a hand mixer, beat the egg whites and a pinch of salt until you have soft peaks. Gradually add 1 1/2 teaspoons sugar, beating constantly, until whites are glossy with medium-firm peaks.
Use a spatula to fold 1/4 of the egg white mixture into the chocolate mixture to lighten it. Then fold the remaining whites into the chocolate until just incorporated, being careful not to overmix or deflate the mixture.
Divide mousse among 4 bowls. Cover and chill until set, about 4 hours (and up to 1 day in advance).
when ready to serve, spoon whipped cream over mousse and sprinkle with chocolate shavings.