Really? Anything with baking powder/soda/yeast... I'm not sure if it's that simple. Baking is all about chemistry. I would make the full recipe and stick the other half of the cake in the freezer, well wrapped. I did an extra cake back in September when I baked BIL's. It was sooo nice just to grab it out of the freezer for DH's birthday yesterday and slap some cream cheese icing on 'er!
ETA: Just did a bit of googling for you...
http://recipes.howstuffworks.com/too...e-a-recipe.htm
http://allrecipes.com/help/recipeinfo/scaling.aspx