Melt chocolate and butter in the top of a double boiler over simmering water until smooth, about 5 minutes; remove from heat and allow to cool slightly. Whisk egg yolks into chocolate one at a time, allowing each yolk to blend completely before adding the next.
Beat egg whites in another bowl until foamy. Beat in cream of tartar. Gradually beat in 1/4 cup white sugar until soft peaks form. Lift your beater or whisk straight up: the tip of the peak formed by the egg whites should curl over slightly.
Beat 1/2 cup heavy cream in chilled bowl until thickened. Add 2 tablespoons white sugar and 1/2 teaspoon vanilla extract; continue to beat until soft peaks form.
Gently fold egg whites into chocolate mixture. Fold whipped cream into chocolate mixture until mousse is just blended. Spoon chocolate mousse into pie crust. Refrigerate until ready to serve.
Beat 1 cup heavy cream, 1/3 cup white sugar, 2 tablespoons confectioners' sugar, cocoa powder, 1/4 teaspoon vanilla extract, and salt in bowl until soft peaks form. Spoon over chocolate mousse before serving.