Yield: 900 mL
Beat the milk and egg yolks together. Blend in the sugar.
Cook the custard over medium heat, stirring constantly with a wooden spoon, until it is thick enough to coat the back of the spoon. This should happen at about 74°C F - 77°C F. If the custard starts to fill with lots of tiny flecks, you've cooked it too far - stop! Remove the custard from the heat. Sift the cocoa into it while beating it, and continue to beat until well blended. A stick-blender is handy for this step. Cool the custard to refrigerator temperature, then add the cream and vanilla. Mix well.
Freeze using your method of choice.