Place chocolate chips and butter in a large glass bowl; set aside.

Whisk together sugar, cornstarch, cocoa powder, and salt in a medium sized saucepan. Gradually whisk in about 1/2 cup of the milk, then the egg yolks. Whisk till very smooth, then add the remaining milk.

Cook and stir mixture over medium heat till it comes to a boil. Turn to medium low and boil at least one minutes, stirring constantly. Remove from heat and add vanilla.

Pour hot pudding through a strainer into the chocolate and butter. Stir gently for several minutes till butter and chocolate are melted and pudding is slightly cooled.

Pour pudding into serving dishes and place plastic wrap over the pudding.

Serve warm, or chill for several hours. Before serving, top with sweetened whipped cream.

For whipped cream, beat cream till soft peaks form. Beat in powdered sugar and vanilla till stiff peaks form.