This cake is very delicate and the frosting is very thick. I recommend you freeze your cake layers before frosting, otherwise your cake will be sticking to the frosting and falling apart. Personally, I find it makes everything easier to make the cake and frosting at least a day in advance and simply assemble the day you need it.
When I make this cake, I’m not particular about how much black cocoa powder I use. I measure out the total dutch + black cocoa powder needed: going most of the way with the dutch-process cocoa powder and leaving the last handful of grams for the black cocoa powder. A little goes a long way for darkening up your cake and frosting. I think this makes all the math easier when I increase the recipe size.
I recommend baking this cake as 2 layers. The frosting isn’t super stable and you might experience sliding or squishing with 3+ layers. Or try using a more stable frosting between the layers.
Dutch-processed cocoa powder can not be substituted for unsweetened cocoa powder as they have different acidities, and that will affect the chemistry of the cake. (And the flavor is also much better)
Black cocoa powder gives the cake and frosting a deep dark color. Only a little bit is needed. It can be replaced 1:1 with dutch-process cocoa powder, however your cake and frosting won’t be as dark. It also has an Oreo-like flavor that partially contributes to this cake.
Droste Dutch-Processed Cocoa Powder or
Rodelle Dutch-Processed Cocoa Powder
The quality of your cocoa powder will determine the flavor of your cake.
Wincrest Black Cococa Powder
Cake recipe is adapted from King Arthur Flour