I always go with half the amount of eggs, to rest of ingredients. i.e. 2 eggs - 4oz of everything else. 3 eggs - 6 oz of everything else. I've never had a problem with it. If I want to add cocoa, I usually take it out of the flour weight.
The recipes are you using must be flawed or you are over mixing the flour in the sponge always put the flour in last and mix by hand by holding. Once flour has been over mixed it releases gluten hence heavy cake. If your oven temp is incorrect you will burn the edges and then have a peak. Make sure you use a good quality tin.
This is one of the best cakes for vanilla - [[ http://www.bbcgoodfood.com/recipes/10860/easy-vanilla-cake]]
I'm wanting to make a rainbow cake for my DP's 40th on Wednesday. Does anybody have a good sponge recipe? Whenever I make sponge it's a bit heavy and dense, nothing like the sponge you get in shop bought cakes. How do I make it soft and light? Thanks