This is my asp:button code which is not rendering font awesome\'s icon but instead shows the HTML as it is:
You can do it, jut not purely with CSS. You just need to set the Text property on the button to the unicode value of the fontawesome character and give the button the 'fa' css class so it takes up the fontawesome font.
I made this helper library that provides all the icon codes strongly-typed if that turns your crank:
Nuget: Install-Package FontAwesome-ASP.NET
Source: https://github.com/kemmis/FontAwesome-ASP.NET