This is a useful calculation when you have no cocoa handy, and let’s face it if you don’t have chocolate handy at all times there’s something wrong with you and maybe therapy and not baking should not be your first priority.
Dark chocolate works best because of the fuller, richer flavour.
For every 1 Tablespoon of regular Dutch-processed/Alkalised Cocoa in the recipe, add 30g melted, cooled dark chocolate and subtract 1 rounded Tablespoon of sugar and 1 level Teaspoon of margarine or butter (because the chocolate contains sugar and fat in addition to cocoa and other stuff).
1 Tablespoon Cocoa = + 30g melted Dark Chocolate – 1 rounded Tablespoon sugar – 1 level Tablespoon margarine or butter
30g melted Dark Chocolate = 1 Tablespoon Cocoa + 1 rounded Tablespoon sugar + 1 level Tablespoon margarine or butter