I can't seem to find the reference for this advice, I suspect it might be Bakewise (book) but as I got it out of the library can't double check. But in any case, I seem to remember reading that if your oven is on too high a temp, the cuppies will rise up quickly, but not have enough structure to maintain that dome once they have cooled. I'm starting to doubt my memory though, after reading the tips here:
CAKE SINKS IN THE CENTER -
- batter over mixed
- too much fat and/or sugar or leavening
- not enough liquid
- oven temperature too low
Read more: http://www.joyofbaking.com/ButterCakeTroubleshooting.html#ixzz1leooszKw
While searching for my source of the above, I found a few other things which back up what fromscratchsf said about your addition of fat being the problem. This is verbatim from another forum which no longer exists, so I can't point you to the post, but I copied & pasted it into a word doc:
"Basically, your recipe is failing because you do not have enough of what is called a structure builder.
(Prepare for the baking chemistry lesson!)
In any recipe, proper balance mus tbe achieved between structure builders, tnederizers, moisteners and driers.
Structure builders are the ingredients that ensure your cake will hold its shape once cooked and cooled.
The cake structure is formed when all 4 components interract chemically, both while in the mixing bowl and while being heated.
A minimum amount of structure is required for all cakes, but too much and you will have a tough end result. However, too little, and you will not have enough structure to hold the cakes shape upon cooling, which is the issue you have epereienced."
Also from one of Lindy Smith's books (regarding the conversion of cake recipes to cupcakes)
"You can use any sponge cake recipe to bake cupcakes, but to avoid domed tops you will need to reduce the amount of flour used. As a general rule, reduce the flour by 15ml (1tsp) for every 150g (5oz) of flour in the recipe."
Hope some of that helps!