In a small saucepan over low heat, melt the butter and chocolate together, stirring until smooth. Remove from heat and let cool to lukewarm.
In a small bowl, whisk egg yolks together until smooth. When the chocolate has cooled to lukewarm, whisk in the egg yolks. The mixture will become very thick and the chocolate will seize - this is normal.
Add 2 tablespoons of hot water and whisk vigorously. Continue to add 1 tablespoon water at a time until chocolate mixture becomes smooth (will still be quite thick). Set aside.
In a large bowl, beat the egg whites with ¼ cup powdered Swerve, the salt and the cream of tartar until they hold stiff peaks. In another bowl, beat the cream with the remaining sweetener and 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. Fold in the whipped cream thoroughly but gently.
Divide amont 6 dessert dishes and refrigerate at least 2 hours.