I would just do it the way I make hot chocolate with dairy milk. Mix the cocoa powder with the sugar, very thoroughly. Then mix in a small amount of the milk and stir until you have a paste and all the cocoa is wet. I prefer to do this in a glass cup, so I can see that all the cocoa has gotten dark and wet. Then I heat the milk, usually in the microwave, and add it to the cocoa paste and stir. It never lumps that way.
May you live as long as you wish and love as long as you live.
Robert A. Heinlein