Though I can see an accepted answer and other great answers too but thought of sharing what I did to solve this issue (in just one line).
CSS ( Created a Class ) :
.circle {
border-radius: 50%;
}
HTML (Added that css class to my button) :
<a class="button circle button-energized ion-paper-airplane"></a>
So Easy Right ?
Note : What I actually did was proper use of ionic classes with just one line of css.
See Result your self on this JSFiddle :