Make a double boiler by setting a medium metal bowl over a small saucepan containing 2-3 inches of water over low heat. Be sure that the bottom of the medium bowl is not touching the hot water, as this would defeat the purpose of the double boiler, which is preventing your chocolate from splitting by heating it very gently. Heat the water to a simmer, then add the chopped chocolate, butter, and coffee to the bowl. Melt chocolate and butter into coffee. Remove from heat and let sit until the melted chocolate mixture is lukewarm.
Put some ice and cold water in a large bowl. Be sure that the bowl that you will use in the next steps fits inside the large chilled bowl while maintaining contact with the ice water and without risking water splashing into the bowl.
Take a fresh metal bowl, and place the egg yolks, sugar, rum, and water together over the saucepan of simmering water. Beat with an electric handmixer until the mixture is thick and pale yellow. Remove the egg yolk mixture to the bowl of ice and beat until they are room temperature and further thickened. Remove from the ice and dry off your ice water bowl - you'll want to use it to beat your egg whites.
Gently fold the chocolate mixture into the egg mixture.
In the dry large bowl, or in the bowl of a stand mixer using the whip attachment, beat the egg whites with a pinch of salt until frothy. Continue beating until they start to hold shape, then add 1 tbsp sugar. Continue beating until thick and shiny with soft peaks, then add the vanilla.
Fold the whipped egg whites into the chocolate base in thirds. You will want to fold until there are no more visible streaks, but try not to overmix or your mousse will lose some its aeration.
Spoon or pipe the chocolate mousse into small cups or ramekins. Let sit until set, at least 4 hours.
Make the whipped cream topping by beating all ingredients together in a stand mixer with a whisk attachment or using an electric mixer. Spoon or pipe the whipped cream topping over the chocolate mousse. Top with chocolate shavings, a dusting of cocoa, or all by itself!