In a medium saucepan whisk together sugar, cocoa, cornstarch and salt.
On low heat whisk in milk one cup at a time until all combined.
Slowly add eggs yolks whisking all together until smooth.
Turn heat up to medium and continue to whisk until the mixture just starts to boil.
Turn back down the heat to low and continue to whisk until pudding thickens. Approx. 2-3 minutes.
Remove pudding from heat and stir in butter and vanilla until butter is melted and blended into pudding.
Pour hot pudding into individual serving dishes. Cover with plastic wrap and chill for 2-3 hours.
