In a saucepan, whisk together the milk, sugar, cocoa powder, cornstarch, egg yolks, vanilla, and salt. Cook over medium-high heat whisking constantly, until the mixture comes to a simmer. Add the chocolate and butter.
Place the saucepan over medium heat and cook, stirring continuously with a whisk or silicone spatula. Be sure to scrape the bottom and sides of the pan to prevent sticking or scorching.
After several minutes, the mixture will begin to thicken. Continue cooking until it reaches a thick, pudding-like consistency, about 8–10 minutes. You'll know it's ready when it coats the back of a spoon and holds its shape when swirled.
Pour the pudding into 6 small cups or ramekins. Place a piece of plastic wrap directly onto the pudding and refrigerate for at least 4 hours or until set.
Video
Notes
Prevent Lumps: Make sure to whisk the mixture constantly to prevent any lumps from forming.
Richness: For an extra creamy texture, add an additional tablespoon of butter to the pudding mixture.
Pudding Too Runny?: If your pudding isn’t thickening as expected, try increasing the heat slightly, but make sure to stir continuously.