Ah, it is not my secret to tell. I must direct you to the lovely blog of Robyn Stone. (I know, shame on me for directing you off Medium, but I must give credit where it is due!) There are many iterations of this chocolate cake on the 'net, but hers is the one I found first years ago when I needed a chocolate cupcake recipe for my kiddo's birthday.
This measurement is for cupcakes, but I have used the same measurement, or sometimes 1.5 the measurement for a cake in a 8.5 x 13 inch pan. The secret is the boiling water. It seems to keep more moisture in.