Place first 3 ingredients in a medium bowl. Microwave at high 1 minute. Stir and microwave 1 more minute or until melted. Stir well.
Combine cornstarch and next 3 ingredients in a medium saucepan. Gradually whisk in milk, cream, and egg yolks. Stir in melted chocolate. Place over medium heat. Cook, stirring constantly, 18 minutes or until mixture begins to thicken. Cook, stirring constantly, 2 more minutes.
Remove from heat, and add butter, stirring until it melts; stir in vanilla. Strain pudding into a bowl. To prevent a skin from forming atop pudding, cover with plastic wrap, gently pressing directly on pudding. Cool 30 minutes. Chill thoroughly.
*In testing, we used Ghirardelli.