I've made several of these over the last couple of days. I maybe repeating some of Zukerig's comments, but they are important. Some important things to consider:
warm the eggs and sugar
make sure to whip them sufficiently (at full speed on a kitchenaid, maybe 10-12 minutes). the volume of the eggs will rise to its maximum and then begin to fall slightly. at that time, put the mixer into "low" and allow it to go for another minute or so. this helps to stabilize the eggs.
fold your flour well but quickly, and be careful in adding your melted butter.
when cooling the cakes, cool them upside down in the pan.
Here's my recipe for 2x8" cakes:
8 oz. sugar
2 oz. cornstarch
6 oz. cake flour
1 1/4 oz. melted butter
375 in a convection oven until set and lightly golden on top.