In a large saucepan, combine the sugar, cocoa, cornstarch and gelatin, pressing out all lumps. Add the half-and-half, evaporated skim milk and whole milk; whisk vigorously to blend. Set the pan over moderately low heat and cook, stirring constantly with a wooden, until the mixture just boils and is thickened and smooth, about 10 minutes. Blend a little of the hot mixture into the beaten egg yolks, then stir the warmed egg yolks back into the pan. Cook over moderately low heat, stirring constantly, for 3 to 4 minutes. The mixture must not boil or the eggs will scramble. Remove from the heat and stir in the vanilla. Set the saucepan on a rack to cool for 10 minutes, stirring frequently to prevent a skin from forming on the surface. Ladle the mixture into 6 decorative 4-ounce pot de cr�me cups or white porcelain ramekins and let cool completely. Cover each cup with plastic wrap, then refrigerate at least 5 hours, or overnight. Serve as is or top each serving with a little of the whipped cream and either a scattering of chopped pistachios or a single candied violet.