Preheat the oven to 350 degrees F. Coat an 8 1/2-inch spring form pan with cooking spray and line the bottom with parchment or wax paper. Set aside. Melt the chocolate in the microwave, double boiler or bowl over a saucepan of simmering water. Meanwhile, separate the yolks from the whites and transfer the yolks to a mixing 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. Sift the remaining 2 tablespoons of confectioners' sugar over the mousse pie just before serving (use a fine mesh sieve if you don't have a sifter).