In a small saucepan, combine 2 cups (480ml) of milk, sugar, cocoa powder, and salt. Whisk until well combined. Cook over medium heat until very steam and just starting to simmer, stirring frequently.

Meanwhile, in a large mixing bowl, whisk together the egg yolks, cornstarch, and remaining ¼ cup (60ml) of milk.

When the chocolate milk mixture is hot, slowly whisk about a cup into the egg yolk mixture. (This tempers the eggs to keep them from scrambling.) Whisk in the remaining hot milk. Pour the mixture back into the saucepan.

Place the saucepan back over medium heat and cook stirring constantly until the mixture bubbles. Continue cooking until thickened enough that the whisk leaves a trace when stirring in the mixture, about 3 minutes. (The temperature should also be about 170°F on an instant-read thermometer.) Remove from the heat and whisk in the butter and vanilla until melted and combined.

Transfer the pudding to a bowl and press plastic wrap to the surface. Refrigerate until chilled, about 3 hours, or up to 4 days. Give it a good stir before dishing into serving bowls. Serve with whipped cream, if desired.