Take the cocoa powder in a cup and add the hot water to make it into a paste. Only use as much water as is needed to make it into a paste. Let it cool. No need to refrigerate.
In the bowl of a stand mixer take the cream, the condensed milk, sugar, vanilla. Always store the condensed milk and cream in the refrigerator, till you use it. The general advise is to keep the beaters and bowl to be used in the freezer. But seriously, I never did all that.
Once the cocoa paste is cool, add it to the cream mixture and mix well till it all blends well.
Taste it and add sugar if you want more sweet. Adding powdered sugar is preferable as it reduces the heat that will be produced when mixing. But I always use granulated sugar.
Whip this mix using a balloon whip in the stand mixer at 6 for 5-7 minutes. That is all you have to do. Just stand back and enjoy looking at the liquid turning into thick fluffy cream.
Transfer to containers and freeze for atleast 6 hours before serving.