How to Make It
Mix sugar, cornstarch, and salt in a medium saucepan. Slowly whisk in milk and bring to a boil, whisking frequently, over moderate heat until thickened, about 8 minutes.
Place cocoa powder in a mug and pour a ladleful of the hot milk over it, stirring well until the mixture is smooth. Add it back to the milk in the saucepan, whisking to create a smooth sauce.
In a medium bowl, beat the egg yolks well. Gradually stir 1 cup of the thickened cocoa-milk mixture into the eggs and then pour the egg mixture back into the saucepan. Bring to a simmer over moderate heat, stirring constantly, then lower the heat and simmer for 1 to 3 minutes until it's thickened.
Remove saucepan from heat and whisk in the vanilla. Pour the pudding into a large serving bowl or 6 small cups or ramekins. Cover and chill for a few hours or overnight.
For topping: Whip heavy cream until peaks form. Sift sugar and cocoa over cream and beat mixture until firm. To serve, spoon cocoa cream over pudding.
How Kids Can Help: Sift sugar and cocoa over cream.