In a medium saucepan, mix the agar agar and sugar with 500ml water. Put on the stove and bring to a gentle boil over low heat, stirring constantly.
Gradually add the cocoa powder mixture followed by the evaporated milk. Mix until well blended and be careful not to let the milk curdle. When the mixture is boiling, remove from the heat and strain into rinsed jelly moulds.
Cool before chilling in the refrigerator to set completely, at least 4 hours. Cut and serve as is or with custard and fruit cocktail.