In order to add moisture to a cake/muffin mixture I would not be looking to add more water for a couple of different reasons:
- The addition of extra water might have an adverse effect on the finished product both due to making the batter runnier, and the addition of extra steam (when the water heats up) may result in a completely different texture when finished (due to hot steam trying to escape). I'm not sure but it may also make the muffins/cakes less stable once they are taken from the oven and the steam trapped inside cools down.
- An amount of the extra water that you incorporated may well evaporate during cooking so its inclusion might be superfluous.
- Once the cake/muffin is finished water will continue to evaporate when it is cooling down, placed in the fridge (especially since fridges cool by evaporation) or stored in anyway. This could cause the finished product to dry out...and you would be back to square one.
I would be looking at adding some vegetable (or other neutral flavoured) oil or butter. You could also use sour cream, however the acidity would of course change the taste of the cake/muffin and if using a chemical leavening agent (ie. baking powder or soda) then it could have an adverse affect on their ability to leaven the product.
Those are my ideas, but I am still a fairly big baking novice, just haven't done enough of it.
