Oh my, the dry chocolate cake thing… There are a couple of threads on this, but basically, first check that you’re weighing everything and covering the cocoa/hot water mixture and not overbaking. If you’re still getting dry chocolate cake, the problem could be a low-fat cocoa, such as Hershey’s Special Dark or Equal Exchange (two dutched cocoas that are low in cocoa butter).
Rose recommends dutched cocoas, which are frequently, but not always, full-fat cocoas. When cocoa powder is made, the less expensive types generally have some of the naturally ocurring cocoa butter removed, while the higher-priced, dutched cocoas retain their cocoa butter. I find this makes a big difference in cakes, with the low-fat cocoas leading to dryness, imbalance of flavor, and in the case of sponge cakes, lower height/ fallen cakes.
One way you can tell this is happening is if you are carefully weighing both the cocoa and boiling water but still finding that the resulting paste is too thick to whisk- more like heavy toothpaste than soft buttercream. Another way to tell is by emailing the company to get the (naturally ocurring) fat content per 100g. Unfortunately, the serving size on nutritional labels is generally too small to tell if the cocoa is low-fat due to rounding errors. Full-fat cocoas are about 20-25% fat by weight, while low-fat ones hover around 10%.
My favorite fix, if buying a different cocoa isn’t an option, is to substitute a little unsweetened chocolate, which is about 50% cocoa butter, to correct the fat content. I find that 75% cocoa and 25% unsweetened chocolate by weight usually works well (that is, if a recipe calls for 100g of cocoa powder, use 75g low fat cocoa and 25g unsweetened chocolate). The unsweetened chocolate needs to be chopped finely and melted in the boiling water along with the cocoa powder.
Dutched is preferable over natural because of its darker color and richer, less bitter flavor, but I don’t think dutching has much influence on whether it’s a dry cake.