I am based in the USA and am planning to cook the Chocolate Raspberry Heart cake from Feast (p139) but wanted to clarify what you mean by 'best unsweetened cocoa'. Is this just plain old cocoa powder? The reason I ask is that I recently made the Chocolate Orange Cake (also from Feast, p274), which also called for unsweetened cocoa. I used Trader Joe's unsweetened cocoa powder and the cake came out considerably lighter in colour than the picture but quite bitter. Should I be using one of the Dutch processed cocoa powders - e.g., Droste's Cocoa for baking and drinking? Alternatively, I could buy Ghirardelli premium baking cocoa. Thank you!
There are two types of unsweetened cocoa powder. Dutch processed (or alkalized) powder is treated with an alkaline to reduce the acidity of the cocoa, giving it a milder and smoother flavour and a darker colour. Natural cocoa powder has not been treated and tends to be more bitter. The actual quality of the cocoa powder is more influenced by the type and quality of the beans used.
The UK doesn't differentiate between Dutch processed and natural cocoa in recipes - both types are available though you will need to read the ingredients label to see if an alkaline has been used. The choice of type of cocoa powder tends to be more of a personal taste, some feel that natural cocoa is frutier and perhaps more chocolatey whilst others feel that Dutch processed cocoa is more complex.
The Chocolate Orange Cake in Feast uses oranges that are simmered in water until they are soft. The oranges are used whole so they include the pith which can contain a slight bitter note, even after the oranges have been cooked. It may be that this, plus the natural cocoa, caused the cake to be slightly more bitter than you expected. However if you are sensitive to bitter flavours then you may prefer to use a Dutch processed cocoa in the Chocolate Raspberry Heart. Ghirardelli is a good brand though we believe that their unsweetened cocoa is the natural type. Valrhona and Green & Black's are two good European brands available in parts of the US and are both Dutch procesed.
The one point to make when using Dutch processed cocoa is that it is not as acidic as natural cocoa so if the cake batter only contains baking soda (bicarbonate of soda) as the leaveneing agent then some additional acid may be needed (such as a pinch of cream of tartar) to help the cake to rise. The Chocolate Rasperry Heart Cake should rise with Dutch processed cocoa as the batter is also made with milk, which is slightly acidic.