In a heatproof bowl set over, but not touching, a pan of barely simmering water, stir the chocolate until melted and smooth. Remove from the heat and whisk in the egg yolks, olive lol, warm water, and salt until well blended.

In a clean bowl, using an electric mixer set on medium-high speed, beat the egg whites with the cream of tartar until frothy. Add the sugar and continue beating until the mixture forms soft peaks. Fold about one-third of the egg-white mixture into the chocolate mixture until no white streaks remain. Gently fold in the remaining egg-white mixture until well incorporated.

Spoon the mousse into 4 custard cups, dividing it evenly, and refrigerate until well chilled, at least 4 house or up to overnight.

Sprinkle with the chocolate shavings. Serve right away.