1. To melt the chocolate, chop and place it in a heatproof bowl set over a pan of almost simmering water, stirring once or twice until smooth. You can also melt the chocolate in a microwave at 50 percent power for 3 1/2 minutes, stopping to stir after 2 minutes. If the chocolate is not yet completely melted, heat up to 30 seconds more at 50 percent power. After melting, allow to cool slightly.




2. Sift cocoa powder, cornstarch, sugar, and salt into large heavy-bottomed saucepan. Slowly whisk in light cream, followed by yolks, then milk. Stir in chocolate. (Chocolate will form clumps that smooth with cooking.)




3. Bring mixture to boil over medium-high heat, stirring constantly with whisk, scraping bottom and sides of pot. Pudding will gradually darken and thicken. Reduce heat to medium and cook, stirring gently but constantly with wooden spoon until pudding very thickly coats spoon or instant-read thermometer registers about 200 degrees, 1 1/2 to 2 minutes.




4. Pass pudding through fine-mesh strainer into medium bowl, pressing with rubber spatula. Leave residue in strainer. Stir butter and vanilla into pudding. Serve warm or directly cover surface of pudding with plastic wrap, cool 30 minutes, and refrigerate.