Instructions

1. Combine chocolate and rum in a small pot; nestle it inside a larger pot filled partway with boiling water. Cover smaller pot and set aside to let melt.
2. Beat yolks in another small pot until pale and frothy. Combine sugar and coffee in another pot; cook over medium heat until dissolved, 5–6 minutes. Pour into yolks in a stream, while whisking; set aside. Pour water into a large pot to a depth of 2". Heat over medium-low heat until hot but not simmering. Nestle pot containing yolk mixture over pot and cook, whisking vigorously, until thick and creamy, 8–9 minutes. Transfer yolk mixture to a clean bowl; beat with an electric mixer until cool, about 5 minutes. Uncover chocolate mixture and stir; add butter and whisk until smooth. Fold chocolate–butter mixture into yolk mixture; set aside.
3. Beat egg whites in a bowl until just frothy. Add cream of tartar and salt; beat to stiff peaks. Stir one-fourth of the egg whites into chocolate–yolk mixture; gently fold in the rest. Spoon mousse into 6 serving cups or dishes; cover; chill until set. Beat cream to stiff peaks; transfer to a pastry bag with a star tip. Pipe a rosette of cream onto each mousse.