Meanwhile, in a medium saucepan off of heat, whisk together sugar, cocoa powder, cornstarch and salt. Very gradually whisk in milk to dissolve cornstarch. Whisk in egg yolks. Cook over medium-high heat, stirring constantly, until the first large bubble forms and sputters. Reduce heat to low and cook, still stirring, cook until pudding is thick, 2-3 minutes more. Remove from heat and pour pudding into a medium bowl (if it has lumps, force it through a fine mesh sieve into the bowl).