For the chocolate pudding: In the top of a double boiler, combine the cornstarch, sugar, and salt. Whisk well until there are no longer any lumps. Pour the milk into the dry ingredients and whisk well. Cook over medium heat, stirring occasionally, until thick and smooth, 12 to 15 minutes. Remove from the heat and add the semi-sweet chocolate and vanilla. Whisk until silky smooth. Strain through a fine-mesh sieve and set aside.
For the chocolate mousse: In a small, medium saucepan, combine the milk and mint leaves. Cook over low heat just until the mixture comes to a low boil. Remove from the heat and infuse for 15 minutes.
Place the bittersweet chocolate in a bowl and strain the milk into the bowl. Let stand for 5 minutes. Whisk until smooth. Let the chocolate cool to the touch, about 10 minutes. Whisk in the egg yolks and set aside.
In a deep bowl, beat the cream until soft peaks form. Fold into the melted chocolate. Beat the egg whites with the sugar until stiff, glossy peaks form. Fold into the chocolate mixture just until blended. Set aside.
For the chocolate custard: In a heavy, medium pan, heat the cream over medium-low heat just until it comes to a boil. Put the chocolate in a bowl and pour the hot cream over. Let stand for 5 minutes, then whisk until smooth.
In a small bowl, whisk together the egg yolks and sugar until a slowly dissolving ribbon forms on the surface when the whisk is lifted. Gradually whisk in the chocolate mixture until smooth. Return to the pan and cook, stirring constantly, over medium-low heat until the mixture coats the back of a spoon. Remove from the heat. Press a sheet of plastic wrap directly on the surface of the custard and set aside.
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