ISO: Flat Cake - Still need help

thanks for the info Cathy, very helpful. But I still have a few more questions.

The cake I am baking has a very light interior-almost angel cake like. It calls for me beating the egg yolk separately from the egg white. And I do it for a long time. Then I add the rest of the stuff like flour to the yolk part then the yolk part is stirred into the whipped egg white part.

Is beating either the egg yolk part and/or the white part too long causing the problem? I thought the white needed to be whipped to give it the softness inside.

What about baking powder, doesn't it give it the light fluffy inside?

Does it matter is the egg and the butter are at room temperature or not when in use?

Does it matter if I stir in the egg yok into the white or the white into the yolk instead?

As you can tell, I am a novice at this.

Thanks.