Melt chocolate and butter in the top of a double boiler over barely
simmering water or in a small bowl set over a saucepan of just-simmering
water. Remove from heat.
In a medium-size bowl, whisk cornstarch, cocoa and 1/4 cup of the
sugar into 1/2 cup of the milk.
In a medium-size heavy-bottomed saucepan, combine remaining 3 cups
milk and 1/2 cup sugar and bring just to a scald over medium heat.
Whisk the hot liquid into the cornstarch mixture, then return the
mixture to the saucepan and cook, whisking constantly and scraping
the bottom of the pan, until tiny bubbles boil up for 10 seconds.
Remove from the heat. Put the eggs and egg yolks into the medium
bowl. Whisking gently, but constantly, slowly drizzle 1 cup of the
hot liquid into them. Return everything to the saucepan and cook,
whisking constantly and scraping the bottom of the pan, until tiny
bubbles boil up for 3 seconds. Remove from the heat and stir in
Strain pudding through a fine sieve into a bowl. Thoroughly stir
in the melted chocolate mixture.
Divide pudding among 8 serving bowls. Place a piece of wax paper
directly on the surface of each one to prevent a skin from forming.
Let cool for 2 to 3 hours, then refrigerate until chilled.