The carbonation will provide a little of the missing fizz from the baking powder but not a lot, so this trick works better ...

This difference means you'll need to adjust your recipe to account for the missing cocoa ... Incorporating cocoa powder is straightforward for baking cookies, cakes, or muffins.

If you add baking powder and still end up with a flat cake, check the best before date. Add eggs and other liquids to the mixture slowly to avoid curdling, which can cause heaviness and a flat cake.