I have some dark baking chocolate, can it be used instead of cocoa powder for the Old Fashioned Chocolate Cake recipe? What quantity would be used as substitute for 40 grams cocoa powder?
Nigella's Old Fashioned Chocolate Cake (from FEAST) has a chocolate sponge that is flavoured with unsweetened cocoa powder. Cocoa powder is made from the residue of cocoa beans, once the cocoa fat (butter) has been pressed out. Baking chocolate will contain a high proportion of similar cocoa solids, but will also contain other ingredients, such as fats (the proportions will vary according to the brand) and possibly sugar (unless it is unsweetened). Therefore we would not recommend switching cocoa powder with baking chocolate. In addition, cocoa powder is very dry and will absorb liquid whereas baking chocolate will not absorb liquid, so the cake recipe would require extra adjustments and we are not able to test those adjustments.
If you have baking chocolate then we would suggest using it in a recipe that already uses melted chocolate, such as the Flourless Chocolate Brownies With Hot Chocolate Sauce or Chocolate Cloud Cake. Both of these recipes would benefit from the deeper taste of baking chocolate and also have enough sugar in them to balance the bitterness of an unsweetened baking chocolate.