Melt chocolate, cream, rosewater, vanilla, and sugar in a heatproof bowl set over a pot with an inch or two or barely simmering water, whisking gently until smooth and melted.
Remove the bowl from the heat. Whisk in the egg yolks, one at a time, whisking well after each addition.
In the bowl of a stand mixer fitted with the whisk attachment, whip egg whites with salt until they form stiff peaks.
Using a rubber spatula, gently fold about one-third of the chocolate mixture into the egg whites, being careful not to deflate them. Fold in the remaining chocolate mixture in two batches just until blended.
Pour mousse into a pastry bag and pipe into prepared chocolate cups. Allow to chill for at least 3 hours or up to 2 days before serving.
Decorate as desired. I used little chocolate decorations, candied rose petals, and pomegranate seeds.