Julia Child's Mousse au Chocolate

Name: Julia Child's Mousse au Chocolate
Prep time: 45 min
Cook time: 15 min
Total time: 1 h
Cuisine: American
Ingredients: 6 ounces 170g bittersweet or semisweet chocolate, chopped; 6 ounces 170g unsalted butter, cut into small pieces; 1/4 cup 60ml dark-brewed coffee; 4 large eggs (separated); 2/3 cup 170g, plus 1 tablespoon sugar; 2 tablespoons 30ml dark rum; 1 tablespoon 15ml water; pinch of salt; 1/2 teaspoon vanilla extract