Step 1 In a medium saucepan over low heat, melt the chocolate with two tablespoons water. Stir to prevent scorching; remove from heat.
Step 2 In another, heavy-bottomed saucepan, whisk together the cornstarch and sugar. Whisk in the milk and melted chocolate. Cook over medium heat, stirring, until creamy, about 3 minutes. Bring to a boil and cook for two minutes, stirring constantly.
Step 3Remove the pan from the heat and whisk in the butter. Temper the egg yolks with some of the pudding by carefully whisking one-half cup pudding into the eggs, followed by a second half cup, to bring up the temperature. Add the yolk mixture to the pan, stirring to fully incorporate.
Step 4Immediately strain the pudding into a medium bowl, and promptly spoon one-half cup into each of 6 serving dishes. Cover with plastic wrap. Refrigerate for at least 2 hours and up to 2 days before serving.