In a small saucepan over low heat, melt the chocolate and butter together, stirring until smooth. Remove from heat and let cool to lukewarm.
When the chocolate has cooled, whisk in the egg yolks. The mixture will become very thick and the chocolate will seize - this is normal.
Add 2 tablespoons of hot coffee or water and whisk vigorously. Continue to add the liquid, 1 tablespoon at a time, until chocolate mixture becomes smooth. Set aside.
In a large bowl, beat the egg whites with the sweeteners, cream of tartar, and salt until they hold stiff peaks. In another bowl, beat the cream with the vanilla until it holds stiff peaks.
Stir a large dollop of the egg whites into the chocolate mixture to help lighten lighten it. Then carefully fold the chocolate mixture back into the egg whites until well combined.
Fold in the whipped cream thoroughly but gently, until no streaks remains. Divide amont 8 dessert dishes and refrigerate at least 2 hours to set.
