How to Make It
In a medium bowl, whisk the sugar with the cornstarch, cocoa and salt. In a large bowl, combine the chocolate and butter.
In a medium saucepan, heat the milk and cream over moderate heat just until bubbles form around the edge of the pan; remove from the heat. Whisk 1/2 cup of the hot milk into the sugar mixture, then whisk in the egg yolks and another 1/2 cup of the hot milk. Return the mixture to the saucepan and cook over moderate heat, whisking constantly, until bubbles begin to form around the edge of the pan and a candy thermometer inserted in the pudding registers 165°, about 5 minutes.
Strain the pudding into the bowl with the chocolate and butter and whisk until incorporated, about 2 minutes. Press a sheet of plastic wrap directly onto the surface of the pudding and refrigerate until chilled, about 2 hours. Scoop into bowls and serve with whipped cream.