Lemon iced cake

[starrychime]

Hoping to make a nice lemon sponge cake. 2 quick questions please.
Should you always used unsalted butter for a sponge cake - recipe just says butter.
Is the rind of a lemon just the yellow outside bit - do you just have to rub it on a grater till you get to the bottom of that layer?

I only ever have salted butter in so always use that. Only the yellow bit not the white pithy stuff as that is yuck.