It turns out that non-alkalized / non-Dutch-process cocoa has a much lower surface energy than good old Hershey’s, to the extent that my Fireball Cocoa Recipe produces powder bombs, even after far more stirring than I’m willing to exert.
The trick is to stir the mud for a while, then let it set for 15 minutes:
That apparently gives the cocoa time to get along with the milk and join forces. Stir it up again, let it sit for a few minutes, then proceed with the recipe: smooooth cocoa with no powder bombs.
A bit more Vietmamese cinnamon is no bad thing, either …