Instructions:
- Prepare the Chocolate: Gently melt the chopped chocolate and butter together over a bain-marie (double boiler) or in short bursts in the microwave until smooth. Stir in the coffee/water and set aside to cool slightly, ensuring it remains liquid.
- Temper the Yolks: In a separate bowl, vigorously whisk the egg yolks and the first measure of sugar (2 tbsp) until pale and thick.
- Create the Base: While whisking constantly, slowly drizzle about one-third of the slightly cooled melted chocolate mixture into the yolk mixture to temper. Pour the tempered yolk mixture back into the remaining melted chocolate and whisk until fully combined. Stir in vanilla/rum, if using. Set aside.
- Whip the Cream: In a clean, cold bowl, whip the heavy cream until soft peaks form. Do not over-whip. Cover and chill immediately.
- Make the Meringue: In a completely clean, grease-free bowl, beat the room-temperature egg whites until frothy. Add the cream of tartar. Gradually beat in the second measure of sugar (2 tbsp) until stiff, glossy peaks form.
- Lighten the Chocolate: Take about one-quarter of the meringue and gently fold it vigorously into the chocolate base (Step 3) to lighten the texture.
- The Grand Fold (Incorporation): Gently fold the remaining meringue into the lightened chocolate mixture in two additions using a rubber spatula, cutting down the middle and sweeping up the side. Be delicate to retain air.
- Fold in Cream: Gently fold in the chilled whipped cream in one or two additions using light, slow movements until just combined and homogenous.
- Chill and Serve: Immediately divide the mousse into individual ramekins or serving glasses. Cover loosely with cling film and chill in the refrigerator for at least 4 hours to set properly.