Sunken Melt-and-mix Cakes
Q: Why is it important to not add hot liquid ingredients to the dry ingredients when using the melt-and-mix method?
A: The melt-and-mix method refers to the mixing technique of heating (and melting) fats, liquids and often sugar before combining with the dry ingredient such as flour, oats and coconut. The main reason that you don't want to heat the mixture too much is that you can prematurely activate the chemical leavener, such as baking powder. Because baking powder is activated by moisture and/or heat if it is triggered by the heat of the melted ingredients before the cake goes into the oven, the chemical leavener can ‘run out’ of leavening ability before the structure of the cake is set, causing the cake to sink before it has finished baking. This is why lots of melt-and-mix cakes often sink badly if you are unaware of this or the recipe doesn't specify to only heat the mixture until the butter melts (using room temperature and diced butter will help it melt at a lower temperature) or to cool the mixture to room temperature before combining it will the dry ingredients containing the chemical leavener.