Epicous the yolks in and of themselves don't make leavening. The action of heating them and whipping them incorporates air into the yolk mixture. The yolks add the strength to hold the air holes (the leavening) when it's baked. Essentially what you then have then without the yolks is an Angel Food Cake!:D
:roll: 'Of an age' what age is that, Rodney? Only joking. For I know what you mean, my age 84, means that though I love food,:talk: I can't eat as much as I used to.:confused: But I love cooking and devising new recipes. My fridge and freezer is full of 'left-overs'. I'd love to go out for a meal sometime but that means my cooking doesn't get eaten up!!
You're wanting a white cake but you want it to be moister and more tender than angel food by the addition of the butter.
I don't think you can do that. The butter is going to deflate the whites and you will lose your leavening. I dont know if you can fold in the melted butter later as you would with a genoise...I tend to think it will still deflate the whites. So you will either need to make a genoise--which includes yolks or find a creamed butter cake recipe that uses whites and uses some form of chemical leavening.