In a double boiler or microwave, melt the 8 ounces of chopped semisweet chocolate along with 2 tablespoons of butter. Stir until smooth and set aside to cool slightly.
In a mixing bowl, beat the cream cheese until creamy and smooth. Gradually add 1/4 cup of sugar and 1 teaspoon of vanilla, continuing to beat until well combined and fluffy.
Once the melted chocolate has cooled slightly, add it to the cream cheese mixture, and blend until fully incorporated.
Separate the eggs, placing the whites in one bowl and the yolks in another. Add the yolks to the chocolate mixture, mixing well.
Using a clean whisk, beat the egg whites until soft peaks form. Gradually add the remaining 1/4 cup of sugar and continue to beat until stiff peaks form.
In another bowl, whip the heavy cream until soft peaks form.
Gently fold the whipped cream into the chocolate mixture. Then, carefully fold in the egg whites, being cautious not to deflate the mixture.
Spoon the mousse into serving cups or ramekins and refrigerate for at least 2 hours, or until set.
