Place the chopped chocolate in a medium bowl and set aside. In another medium bowl sift together 1/2 cup sugar, the cornstarch, cocoa, and salt. Whisk to combine and then stir in 1/2 cup milk to make a smooth paste. Scrape the bottom of the bowl with a rubber spatula to make sure it's completely mixed.
Combine the remaining 1/4 cup of sugar, 1 1/2 cup of milk and 1 1/2 cups of cream in a large pot and scald over medium high heat. Reduce heat to medium low and stir in the cocoa paste. Cook, stirring consistently, but gently with rubber spatula or whisk, until the mixture comes to a boil. Stirring gently, boil on low heat for just one minute.
Remove pan from heat, strain the hot mixture into the bowl with the chopped chocolate. Add the vanilla and stir gently until the chocolate is melted and combined. Pour the pudding in 6 ramckins or parfait glasses, place them on a cooling rack and let sit out at room temperature to cool, about an hour. Cover with plastic wrap and chill for 6 hours or up to 2 days.
Before serving, combine the heavy cream, sugar and vanilla in a bowl and beat until soft peaks form. This can be done up to 6 hours before serving. Refrigerate and rewhisk briefly at the last moment. Top each pudding with a mound of cream and dust with a little sifted cocoa