Put the chocolate, butter, and coffee, in a heatproof bowl and place over a saucepan containing about 1-inch of barely simmering water (or use a double boiler). Stir with a wooden spoon until the chocolate is melted and the mixture is smooth. Remove the bowl from the heat and let cool slightly. Then grab a whisk and beat the egg yolks into the chocolate, 1 at a time, beating until smooth after each addition. Set aside.
In another bowl, beat the egg whites until foamy. Add the cream of tartar and beat until soft peaks form. Gradually beat in ¼ cup sugar and continue beating until stiff peaks form.
In a chilled bowl, beat the heavy cream until it begins to thicken up. Add the remaining 2 tablespoons sugar and the vanilla and continue beating until the cream holds soft peaks.
Gently fold the egg whites into the chocolate mixture to lighten it. Then, fold in the whipped cream. Do not over work the mousse or it will be heavy. Divide the mousse between 4 individual glasses. Cover and chill for several hours. Garnish with chocolate shavings and fresh berries, if using. Enjoy!
