I’ve been having problems with this cake. Barring silly mistakes I still get soft lumps of cocoa/flour in the mixture. The cocoa powder is too much to dissolve all in the boiling water (dissolving cocoa in boiling water intensifies the flavor). I could separate 700 cocoa to dissolve in the water next time, perhaps that will help to reduce the lumps too. I might try portioning out some of the milk to heating up and dissolving the cocoa in.
cream of tartar 90
cocoa powder 1490 (?)
boiling water 2310
Mix cream of tartar with milk and let stand for 5 mins.
Sift flour, baking powder, baking soda and cocoa together.
Beat oil, vanilla and sugar til combined. Add egg. Combine.
Add dries in a few additions, fully combining before adding the next addition.
Add boiling water.