In a medium bowl, whisk together the egg yolks, cornstarch, cocoa and about ¼ cup milk, then whisk in the remaining milk; set aside. In a small bowl, combine the boiling water and miso; whisk until the miso dissolves.

Put the chocolate in another medium bowl and place a fine-mesh sieve across the top; set aside. In a medium saucepan over medium, combine the sugar and butter. Cook, stirring occasionally, until the mixture is bubbling vigorously and thickens, 5 to 7 minutes. Remove the pan from the heat and whisk in the miso mixture; the sugar mixture will sizzle and seize up. Continue whisking until smooth.

Return the pan to medium. Whisk the yolk-milk mixture to recombine, then gradually whisk it into the sugar-butter mixture in the saucepan. Bring to a simmer, stirring constantly; once simmering, cook, stirring vigorously, for another 30 seconds. Pour the pudding into the prepared sieve; push it through with a silicone spatula and scrape the bottom of the strainer to collect all of the pudding. Whisk until smooth, incorporating the chocolate.

If desired, divide the pudding among serving dishes. Cover and refrigerate until chilled, at least 2 hours or up to 3 days.