Just use any cocoa powder (not drinking chocolate powder, you're after something where the only listed ingredient is 'cocoa').

The Dutch Processed bit is not really relevant to the recipe, it's a slight difference in the processing which 99.9% of people wouldn't be able to detect in a taste test anyway. As far as I can tell, it has a slight snob cachet here, but doesn't seem to make a jot of actual difference.

(Especially after they've drowned it in a shuddering amount of sugar; I automatically reduce the sugar by any US recipe by half, and often take it down to one-third on the second making. Except if I'm cooking for US kids - learnt that the hard way after making a chocolate ganache sauce with 70% choc/cream for my son's birthday cake, and found only my kids would eat it...)

kodokan (lives in the US)