When making mousse from scratch, there are two important tips for a successful outcome. First, make sure to thoroughly clean and dry the mixing bowl and beaters before beating the egg whites. Second, room temperature egg whites beat better than cold ones.
Preheat the oven to 350 degrees F. Coat an 8 1/2-inch springform pan with cooking spray and line the bottom with parchment or wax paper. Set aside.
Melt the chocolate in the microwave, a double boiler, or a bowl set over a saucepan of simmering water. Meanwhile, separate the egg yolks from the whites and transfer the yolks to a bowl.
Beat the yolks until thick and pale. Gradually beat in the chocolate until blended. Fold in the vanilla. Beat the whites with 1/4 cup of the sugar until soft peaks form. Fold one-quarter of the whites into the chocolate mixture and then fold in the remaining whites.
Pour the mixture into the prepared pan and bake for 20 to 25 minutes, until almost set but still slightly jiggly in the center. Cool the mousse in the pan before cooling completely (still in the pan) in the refrigerator. Dust with the remaining 2 tablespoons of sugar before serving.
Source: Robin Takes 5 by Robin Miller, Andrews McMeel Publishing