Take a heatproof bowl and place semisweet chopped baking chocolate and unsalted butter in it. Place your bowl over a saucepan, containing about one inch of slightly bubbling water. Stir this mixture with a wooden spoon. Continue stirring until the chocolate melt and the mixture is smooth.
Remove your chocolate mixture from the heat and set it aside until cool properly.
Now crack the three eggs into a large bowl, one at a time. Separate the yolk from the albumin and place them in a small bowl. Pick up a whisk and beat the three egg’s yolk into the chocolate mixture one by one, beating until smooth after each addition. Set this mixture aside.
Take another medium bowl and place the albumin, beating them until foamy. Add 1/2 teaspoon cream of tartar to the albumin and beat until the formation of soft peaks. Now, gradually beat in 2 tablespoons sugar and continue beating until stiff peaks form.
Add the 1/2 cup cold heavy cream in another medium chilled bowl and beat this cream until it starts to thicken up. Add the remaining 2 tablespoons sugar to it along with 1/2 teaspoon vanilla extract. Now, continue beating this mixture until your cream holds soft peaks.
Finally you have the three mixtures prepared, put them altogether in a large serving bowl. Gradually fold the albumins into your chocolate mixture to lighten it. Now, fold it into the whipped cream. Make sure not to over work the mousse otherwise, it will be heavy.
Take four individual glasses and equally divide your chocolate mousse among them. Cover the glasses properly and chill for several hours.
Garnish your glasses with whipped cream and raspberries. Your Chocolate Mousse is ready to serve.