Measuring flour correctly is essential when baking. Too much flour can result in a dry and dense cake, while too little flour can result in a cake that falls apart. To measure flour correctly ...

Overbeating the flour will overwork its gluten ... where the temperature is most likely to be consistent. Too much baking powder can cause the cake to rise rapidly and crack.

This will help protect the outside of the cake from darkening too much. Put the flour, cocoa, mixed spice and baking powder into a bowl with a generous pinch of salt and whisk together.