Melt chocolate: Place chopped dark chocolate in a heatproof bowl set over simmering water, stirring gently until completely smooth and glossy, about 3 minutes.

Whip cream: In a chilled mixing bowl, beat heavy cream on medium-high speed until soft peaks form, pale white and billowy, about 2 minutes.

Beat egg whites: In a clean, dry bowl, whisk egg whites and sugar on high speed until stiff, glossy peaks form, about 3–4 minutes.

Fold: Using a flexible spatula, gently fold whipped cream and egg whites into melted chocolate until no white streaks remain.

Refrigerate: Divide the chocolate mousse evenly among individual glasses, cover with plastic wrap, then refrigerate at least 2 hours until chilled and set.